List of easily available RPGs and a few of their general qualities.
d20-ish Stuff - New school and old school games somewhat related to the d20 system.
Pathfinder - When 3.5 was dropped in favor of major 4th Edition changes, Pathfinder picked up the old thread and ran with it.
Lite variants - M20 and other rules light games, old school and new.
d6 System - Pool our d6 dice, roll and add together. Uses Character Points and Fate Points for a dice boost. Now available in:
Space - General space opera.
Adventure - Modern Era stuff.
Fantasy - Swords & Sorcery, etc.
Mini Six - A lighter, more flexible, universal variant of the d6 system. Mini Six uses Hero Points, which provide a wider range of narrative bumping techniques than the Fate and Character points from other versions.
Donjon - Freeform dungeon-crawling adventure. See also: txt version, character sheet & GM screen, and fancy eBook version.
Dungeon Crawl Classics - Free PDF of the beta version.
Eclipse Phase - Transhuman horror space opera.
Experimental Playground - Home of Savage Flower Kingdom (fantasy), Mutants & Machine-guns (post-apocalyptic), and The Sprawl (cyberpunk).
Fate - More narrative-ish Fudge-like game.
Fudge - Basic and widely used dice pool resolution system.
GORE - A free/open game knockoff of the Basic Role-Playing (i.e. Call of Cthulhu) system. Solid for basic human characters. Not defaulted to include more unusual characters.
GURPS Lite - Your gateway drug to a million setting books.
Lasers & Feelings - Freeform space opera madness (nod to Star Trek).
Sorcerers & Sellswords - Fantasy hack of Lasers & Feelings.
Mutants & Masterminds - Newest version of the system available free as open game content. Point buy and kind of crunchy though.
PDQ (and PDQ Sharp) - Prose Descriptive Qualities. A basic, elegant little system with player defined abilities. Some narrative impact on actions, but not so uch a spendable pool.
PbtA - Powered by the Apocalypse. Games based loosely off the Apocalypse World/Dungeon World engine.
Simple World - The elegant little generic build it yourself version.
World of... - Imagining what Dungeon World would have been like as a minimalist, old-school game from the 1970s/80s.
World of Dungeons - Also in black and white.
World of Mutants - Gamma Worldish edition.
Everything Else - A compilation of various hacks.
Retro Clones, Fantasy - D&D type stuff.
Retro Clones, Sci-fi - Gamma World, Space Opera, etc.
Retro Clones, Other - Supers, other weird stuff.
Risus - Included for the sake of thoroughness. From experience Risus works well for one-shots. Not sure how it'd run for long term play.
Savage Worlds (test drive) - The free quickstart for Savage worlds.
Star Frontiers
Original Rules - I assume this site is legit since the meta-text for the main page claims:
"The first officially blessed site by TSR, Inc."
Remastered Rules - I assume this site is legit since their Star Frontiers main page claims:
"What we have built is a labor of love... we do not profit from it nor can we. We do have a letter from WotC/Hasbro giving us permission to do what we're doing."
Tri-stat/dX - A point buy/build your character system. A crunchier version of the old BESM systems.
True20 (quickstart) - If you like d20 systems, but without all those pesky lesser dice. Not so much with the narrative feeding systems though.
WYRM System - A well packaged but relatively basic set of mechanics. Nice stuff. Three main flavors, plus supplements:
Warrior, Rogue, Mage - The original, fantasy version of the WYRM system. Uses "Fate" points for a narrative boost. Several supplements also.
Resolute, Adventurer, Genius - Pulp version of the WYRM system. Uses "Luck" points for a narrative boost.
WYRED - A Cyberpunk variant. By a different guy.
Main Sequence - A space exploration variant. Spinoff from WYRED. The opposite of polished.
WaRP System - The system from Over The Edge, released as setting-independent open game content. Moderately light system with player defined abilities. Not much by way of story point tokens though.
Wushu - Pretty much a narrative engine for waging bad ass fight scenes.