Castles & Crusades, from Troll Lord Games, is one of the RPG systems utilized with the Irrin setting. It is d2o-based, but with an eye towards being compatible with both older D&D editions and source material as well as current and recent d20 iterations.
I'll let the Wikipedia entry take over for a basic intro:
Castles & Crusades is a role-playing game published by Troll Lord Games in 2004. It was conceived as a reimagining of classic Dungeons & Dragons using streamlined mechanics from third edition Dungeons & Dragons. The game uses many of the d20 System mechanics, but eliminates skills and feats and expands the number of character classes to 13. Another feature is the reversion of some rules to the original versions seen in original Dungeons & Dragons (often referred to as "OD&D") or first edition Advanced Dungeons and Dragons (AD&D).
The system that Castles & Crusades uses is called the SIEGE Engine. A significant new mechanic is the notion of "Primes," often referred to by gamers as the 12/18 system. Through selection of character class and race, players designate some of their character's attributes as "Prime." These Prime attributes use a challenge base of 12, while non-Prime attributes use a challenge base of 18. Tasks based on the Prime attribute are therefore much easier to accomplish than those based on non-Primes. Primes allow a wider variety of characters to be created because characters of same class may excel at different types of tasks.
The first three products produced for Castles & Crusades were an introductory boxed set, the Players Handbook, and Monsters & Treasure, a list of creatures and magic items for the game. A collection of alternate rules and guidelines for modifying the system, the Castle Keepers Guide, is expected in 2008. Troll Lord games has published more than a dozen adventures for the system, and small-press publisher Goodman Games has produced three adventures for Castles & Crusades under license.
The name of the game derives from the Castle & Crusades Society, founded in the pre-Dungeons & Dragons era, by Gary Gygax. The title is in homage to the roleplaying industry's birth.
The shared lineage and resulting similarity in rules between Dungeons & Dragons, Advanced Dungeons & Dragons, HackMaster, and Castles & Crusades simplifies conversion of rules and statistics between the games.
There are multiple books available for Castles & Crusades, but perhaps the most useful for the prospective player is the Player's Handbook. Of course, there's also a free introduction available that contains the rules basics. The Player's Handbook is just now in its 4th printing, but if you find a 2nd or 3rd printing, it should still be largely game-compatible.
If you're looking just to take a look at the rules, you can download the Quick Start version for free. Honestly, though, C&C's rules don't take long to get. After a session or two at the most, everything should largely click into place (if not sooner!). I always recommend learning the rules through actual play.
You can find links to other worthy Castles & Crusades books and pdfs through these companies:
-Troll Lord Games
-Brave Halfling Publishing
The following are useful links I've found pertaining specifically to Castles & Crusades. Feel free to poke around these sites, take note of what other folks are doing with C&C, and bring back any resulting suggestions you may have:
Troll Lord Games Home Page: The publishers of Castles & Crusades and various other gaming products.
Castle & Crusades Society: Keepers of the Society Roles, dedicated gamers, and compilers of The Domesday Book, a fanzine.
Dragonsfoot C&C Downloads: Dragonsfoot hosts a number of C&C files.
Troll Lord Forums: The message board for Troll Lord Games and Castles & Crusades.
Errata, C&C Player's Handbook (2nd Printing): This is a direct pdf link for the errata for the 2nd Printing.
Troll Lord Games Store At RPGNow: PDFs for sale of various Troll Lord C&C products.
C&C Resource Thread: An updated listing of a wider range of C&C resources.
Omote's C&C Stuff
Mythmere's Wondrous Resource
The Grey Elf's Page
Dan's C&C Page
Traveller's Roleplaying Downloads