Game Systems and Documents

Aside from individual .PDFs on /rs/ and massive 15GB torrents on Pirate Bay, there are plenty of gaming resources available online, free and legal.

System Reference Documents

d20 SRD for Dungeons and Dragons 3.5E

Find it here!

The best source for all information and documentation outside of the WoTC site itself, in a much friendlier, easier to navigate and more comprehensive presentation.

Hypertext d20 SRD - d20srd.org

Find it here!

Another awesome source for all information and documentation about the d20 System Reference Documents.

d20 Modern SRD

Find it here!

From the same site and authors as the d20 SRD for D&D 3.5E.

Free Gaming Systems

Risus: The Anything RPG

Find it here!

Risus is a free, casual RPG using a simple and fun "cliche" system of character generation and interaction.

Big Breasts, Small Waist

Find it here! (Locally hosted, HTML)

A riff on Big Eyes, Small Mouth using a system similar to GURPS, BBSW is a goofy hentai RPG. I don't think anyone would ever actually play it, but I have a copy and it's in HTML, so here you go.

FANTASY ADVENTURE TO ADULT LECHERY (F.A.T.A.L.)

Find it here! (Locally hosted, JPG-RAR)

ಠ_ಠ

I really wouldn't, if I were you. Just... look away.

Hybrid

Find it here!

Yeah, okay, I'm pretty much just fucking with you now. Skip down to the next one.

Wushu Open

Find it here! (Locally hosted, Richtext)

The core rules for Wushu, an action-adventure game with a heavy emphasis on crazy kung-fu and over-the-top cinematic glee.

1000 Blank White Cards

Find it here! (Locally hosted, HTML)

All you need is a stack of blank index cards, some pens, and your imagination.

Dvorak

Find it here!

Like 1,000 Blank White Cards, but more complex, with an aim as a testing system for building your own card game.

Call of Cthulhu Quickstart

Find it here!

A quickstart PDF for Call of Cthulhu. Great for reference.

Cthulhu Rising

Find it here!

Futuristic Call of Cthulhu, for "science-fiction campaign set almost 300 years into humanity's future, by which time interstellar travel has become a reality."

Fungeon

Find it here!

Masterless Dungeon Crawling: Each player is both an adventurer AND a DM, taking turns telling the story.

Miscellaneous

Character Sheets

Find it here!

Character sheets for just about every tabletop in existence.