Here is a short list of my favorite free games available out there on the internets.
Dungeon Crawl Classics RPG (Free Beta Version)
Probably my second favorite implementation of D&D. Of course you will want to buy the full final version in hard copy, but in case you don't believe how awesome it is, the beta version should give you a good idea.
Basic Fantasy RPG
A Moldvay Basic D&D clone with some SRD-isms, like ascending AC. Totally free! Also available in OpenOfiice format for hacking, and hard copies are sold on Lulu at cost.
Labyrinth Lord (Free No-Art Version)
Another Moldvay Basic D&D clone, but more true to the original than Basic Fantasy RPG. I consider this to be the defacto standard OSR game. Hard copies can be ordered through Lulu (hardcover or perfectbound) or your local game store (perfectbound only).
Mutant Future (Free No Art Version)
A simulacrum of TSR's Gamma World. Very well put together and tons of fun!
Swords & Wizardry White Box (old version)
Inspired by the original 1974 D&D set, this game is lean and fast. A great choice if you just want to get down to business and start playing, making things up as you go.
Lamentations of the Flame Princess Weird Fantasy Role-Playing (Free No-Art Version)
James Raggi's take on D&D, based on Basic Fantasy RPG.
Delving Deeper (Free No-Art Version)
An attempt to clone original 1974 D&D as closely as possible. Grognard types will argue about how successful they were, but I personally think they did a fine job!
Book I: Adventurer's Handbook
Book II: Referee's Guide
Book III: Monsters & Treasures
Tunnels & Trolls Quick Start
Enough T&T for several adventures worth of fun. A great alternative to D&D.
Simply amazing! Join the yahoo group for access to a nice printable version.
Stars Without Number
A space opera RPG, more or less compatible with Basic D&D, complete with a well-built toolbox for building your own interstellar sandbox to adventure in.
Final Fantasy d6
A fan-made implementation of Final Fantasy based on WEG d6. I haven't played it myself, but it looks really cool.
Adventure Time RPG
This is based on 4th edition D&D, which I honestly think is a horrible choice of system, but the book is beautifully put together, and you can always use it (as I have) as inspiration to craft your own version.