Finite Earths

At the end of 52, a new Multiverse was created and a glimpse was given of some of the worlds it held. As time went on, some of these Earths were mentioned or even explored, principally in CountdownThe Search For Ray Palmer and Countdown: Arena. Below is a list of the Earths that are mentioned and, where possible, some information on them or the people that inhabit them.

Much of the information comes from this story on Newsarama as well as this page in Wikipedia and this page in the DC Database Project. 
New Earth / Earth 0
 - At the end of 52, Rip Hunter referred to "Earths 1 to 51 and New Earth" so a case could be made that New Earth is actually Earth 52. As it hasn't been referred to as such, however, I'm sticking with New Earth as the title of the main DCU Earth that just about every event takes place on. It's the world of Clark Kent's Superman, Bruce Wayne's Batman and Princess Diana's Wonder Woman. In Final Crisis #7, however, it is revealed that the main Earth is called Earth Zero.

Earth-1 - With the publication of Tangent: Superman's Reign, the potential for confusion in the Multiverse grows. While it deals with the Tangent universe (see Earth-9) it clearly labels the Earth of the main DCU, in particular the one where the Justice League of America series is set, as Earth-1. Either this means Earth-1 and New Earth are one and the same (which renders Rip Hunter's line quoted above as incorrect) or this is an error.

Earth-2 - Home to a Justice Society whose Superman and Power Girl are missing. As each of the Multiverse's worlds are new and are not recreations of the pre-Crisis On Infinite Earths worlds, there is - as yet - no reason to believe that Power Girl or Kal-L (as seen in Infinite Crisis) belong to this Earth or are the characters who are missing.  Similar to the pre-COIE Earth-Two. The Justice Society Infinity dwell here, a team that succeeded this worlds JSA.

Earth-3 - Home to the Crime Society which shares many similarities with the Crime Syndicate who live on the Anti-matter universe's Earth and also similar to the pre-COIE Earth-Three. Featured in Countdown #31 where the Challengers From Beyond battle the Crime Society, and in more depth in The Search For Ray Palmer: Crime Society #1. This issue tells the origin of The Jokester and reveals the counterparts of villains on New Earth are actually heroes on Earth-3. Johnny Quick, this world's counterpart to The Flash, featured in Countdown: Arena and won his competition, serving in Monarch's army.

Earth-4 - Home to the Charlton characters Question, Nightshade, Blue Beetle, Captain Atom, Peacemaker and Judomaster. Similar to the pre-COIE Earth-Four and also shares a similarity with the non-DCU world of The Watchmen. The "Quantum Superman" of this Earth - whose appearance closely resembles that of The Watchmen's Dr. Manhattan - took part in the events of Superman Beyond 3D.

Earth-5 - Home to the Shazam characters Captain Marvel, Mary Marvel, Captain Marvel Jr, Mr Tawky Tawny and Mr Atom. Similar to the pre-COIE Earth-S. Captain Marvel took part in the events of Superman Beyond 3D.

Earth-6 - Not yet seen in full though this Earth's Ray Palmer featured in Countdown: Arena. Originally The Atom, he had by the time of his appearance in Arena become The Ray and displayed light based powers. Winning his contest, Palmer serves in Monarch's army. It was briefly glimpsed in Superman Beyond 3D.

Earth-7 - Not yet seen though this Earth's Starwoman featured in Countdown: Arena. She won her competition, almost by accident, and serves in Monarch's army.

Earth-8 - Home to Lord Havok and his Extremists, the world is referred to as Angor by its inhabitants and is similar to the world destroyed by the original Extremists as shown in Justice League Europe #15 - 19. The Challengers From Beyond visited Earth-8 in Countdown and it is shown in more detail in the Lord Havok And The Extremists mini-series.

Earth-9 - Home to the Tangent Universe heroes who, during Infinite Crisis, were seen fleeing from Earth-97. The Flash of Earth-9 appears in Countdown: Arena and, along with many of the other Tangent universe characters, is due to appear in Tangent: Superman's Reign later in 2008.

Earth-10 - Home to the Quality characters, particularly Uncle Sam and the Freedom Fighters, who inhabit a world in which the Nazis won World War II. The Ray of this Earth featured, and was killed, in Countdown: Arena.

Earth-11 - Home to gender reversed characters such as Superwoman, Batwoman and Wonderman all of whom appear, along with other characters, in The Search For Ray Palmer: Superwoman/Batwoman #1. Relations between the female Luthor and Superwoman seem to be friendlier than in most other worlds, though Luthor still refused to help Superwoman in time of trouble.

Earth-12 - Similar to the futuristic world shown in the Batman Beyond cartoons, this world's Green Lantern (a descendant of Hal Jordan) featured in Countdown: Arena. He won his competition and serves in Monarch's army.

Earth-13 - Home to Eve of Shadows and Brigadier Atom, the former of which was a contestant in Countdown: Arena. After winning her competition, Eve left the Arena for her home world where she found Monarch waiting with the Brigadier. In punishment for leaving the Arena, Monarch caused a vast explosion that destroyed much of this world's America. Brigadier Atom was later killed and absorbed by Monarch with Eve forced to serve in his army.

Earth-14 - Not yet seen.

Earth-15 - Shown in Countdown, this world was one where the proteges of the heroes had all taken over the roles of their mentors; Donna Troy was Wonder Woman, Jason Todd was Batman, etc. The entire world was later destroyed by Superman-Prime, however.

Earth-16 - Home of the Super-Sons, this world's Superman featured in Countdown: Arena.

Earth-17 - In a post-apocalyptic world similar to that of the Atomic Knights and Kamandi's world, much of the population is simian. This Earth's Starman - an ape - was featured, and killed, in Countdown: Arena. The world was also glimpsed - with the Atomic Knights on dog-back - in Superman Beyond 3D.

Earth-18 - Home to the Elseworlds story Justice Riders, this world's adventures take place in the Old West where Wonder Woman is sheriff of a small town called Paradise. This Earth's Wonder Woman featured, and was killed, in Countdown: Arena

Earth-19 - The world of the Elseworlds story Gotham By Gaslight, this Earth's Batman is from the 1880's and tackled the Victorian murderer Jack The Ripper. This world's Blue Beetle appeared in The Search For Ray Palmer: Gotham By Gaslight #1 and Batman featured in Countdown: Arena.

Earth-20 - Not yet seen in full, though glimpsed in Superman Beyond 3D with what appeared to be Dr. Fate and Lady Blackhawk.

Earth-21 - The home of the Elseworld's New Frontier series that takes place in the 1950's and which shows the introduction of the Silver Age versions of heroes.

Earth-22 - The Elseworld universe of Kingdom Come. While this Earth hasn't played much of a part in Countdown (at least as of January 2008) it is a key part in the Justice Society of America storyline called Thy Kingdom Come where the Superman of this world, as well as Starman, both find their way to New Earth / Earth-0.

Earth-23 - Not yet seen.

Earth-24 - Not yet seen.

Earth-25 - Not yet seen.

Earth-26 - Similar to the pre-COIE world of Earth-C, this world is populated by intelligent anthropomorphic animals, in particular Captain Carrot and his heroic allies in the Zoo Crew. In the Captain Carrot and the Final Ark! series, Earth-26 is rendered uninhabitable forcing the Captain and his allies to travel to New Earth where they become animals, unable to communicate with the heroes of that Earth. The Scarab - this world's version of Blue Beetle - featured in Countdown: Arena, won its competition and serves in Monarch's army. 

Earth-27 - Not yet seen.

Earth-28 - Not yet seen.

Earth-29 - Not yet seen.

Earth-30 - Home to the Elseworlds Red Son series, the world was briefly visited in Countdown by the Challengers From Beyond before being explored more fully in The Search For Ray Palmer: Red Son. This world's Superman featured in Countdown: Arena and, having won his competition, serves in Monarch's army.

Earth-31 - Home to an older version of New Earth's heroes, in particular a Superman who works directly for the President of the United States, albeit begrudgingly. It is possible, due to its similarities, that this may be the world shown in Batman: The Dark Knight Returns, though this has not been confirmed. This Earth's Superman featured in Countdown: Arena.

Earth-32 - Home to the Elseworlds Batman: In Darkest Knight story where Bruce Wayne becomes Earth's Green Lantern instead of Hal Jordan. This world's Green Lantern is featured in Countdown: Arena.

Earth-33 - This world is home to magical versions of heroes such as Bat-Mage and Lady Flash. It was visited by Forerunner in Countdown To Adventure and the heroes were offered the chance of joining Monarch in his coming war with the Monitors but they refused. This world's Blue Beetle appeared as an agent of Monarch in Countdown: Arena and was killed.

Earth-34 - Home to the Elseworlds Wonder Woman: Amazonia story which takes place in the 19th Century and features a Wonder Woman fighting against a corrupt British Royal Family. This Earth's Wonder Woman featured in Countdown: Arena and, having won her competition, serves in Monarch's army.

Earth-35 - Not yet seen.

Earth-36 - Not yet seen.

Earth-37 - Home to the Elseworlds Batman: Thrillkiller story which is set in the 1960's. This world's Quantum-Storm (an amalgamation of Firestorm and Captain Atom) was featured, and killed, in Countdown: Arena.

Earth-38 - Not yet seen but this Earth's Captain Atom appeared in Countdown: Arena and claims to be the leader of the Atomic Knights. Both the Captain and some (if not all) of the Knights are later killed by Monarch in Countdown: Arena.

Earth-39 - This world has not yet been seen but the Blue Beetle of Earth-39 featured, and was killed, in Countdown: Arena.

Earth-40 - Home to the Elseworld stories JSA: The Liberty File and JSA: The Unholy Three which are set in the 1940's. This Earth's Batman featured in Countdown: Arena where he was bitten and infected by the vampire Batman of Earth-43 and became a vampire himself.

Earth-41 - Not yet seen.

Earth-42 - Not yet seen.

Earth-43 - Home to the Elseworlds Batman: Red Rain stories wherein Batman becomes a vampire, battles and defeats Dracula. The Challengers From Beyond visit Earth-43 in The Search For Ray Palmer: Red Rain #1 and encounter the vampiric Batman. He is featured in Countdown: Arena where, during his competition with the Batmen of Earth-40 and Earth-19 he turns Earth-40's Batman into a vampire. Winning the competition, he serves in Monarch's army.

Earth-44 - Not yet seen, although in Final Crisis #7, there is a brief appearance by the Metal Men of this world and their human leader, "Doc" Tornado. On this world, the Metal Men are robotic versions of the Justice League.

Earth-45 - Not yet seen.

Earth-46 - Not yet seen.

Earth-47 - Not yet seen.

Earth-48 - In this universe, humanity has become extinct and the other races of the solar system use the planet as a War World to settle disputes between themselves. Bred by the Monitors, the Earth is solely populated by the Forerunners of which Viza Aziv is the last. At first a lieutenant of Monarch who aids in the training of his army, she is later discarded by him.

Earth-49 - Not yet seen.

Earth-50 - Home to the Wildstorm characters including Gen 13 and The Authority. The Challengers From Beyond visited the world in The Search For Ray Palmer: Wildstorm #1. Later, Apollo featured in Countdown: Arena.

Earth-51 - As shown in Countdown, this is a world where the heroes succeeded. There is little or no crime on the planet and all the heroes, with the exception of Batman, have retired. After traveling the multiverse following the events of Identity Crisis, Ray Palmer finally settled on this Earth. Following the invasion of Monarch and his multi-dimensional army, Earth-51 and its universe were destroyed. It was later revealed to have been "reconstituted" following its destruction but fell to ruin again when the Great Disaster (shown in Countdown #5 and #4) took place there. The Monitor ship Ultima Thule crashlanded on this Earth during Superman Beyond 3D.