Rules for several games are free, under Wizards of the Coast's d20 license.
Hypertext d20. This includes material from the Player's Handbook, Dungeon Master's Guide, Monster Manual, Epic Handbook, Psionics Handbook and Unearthed Arcana.
Another source of the core 3.5 rules, organized with menus.
D&D 4th Edition
Other d20 systems
Pathfinder, published by Paizo, is an updated D&D game with a huge amount of additional free material. Pathfinder is very popular among people who don't like D&D 4.
Mutants and Masterminds 3rd Edition modifes the d20 rules to let you play a superhero! This is a really fun and flexible system, allowing you to play pretty much anything you can imagine.
Ars Magica, a system with an extremely complex and flexible magic system, where almost everyone plays a mage. They've released their 4th edition rules to the public domain.