Aside from individual .PDFs on /rs/ and massive 15GB torrents on Pirate Bay, there are plenty of gaming resources available online, free and legal.
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.
Another awesome source for all information and documentation about the d20 System Reference Documents.
From the same site and authors as the d20 SRD for D&D 3.5E.
Risus is a free, casual RPG using a simple and fun "cliche" system of character generation and interaction.
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.
Find it here! (Locally hosted, JPG-RAR)
ಠ_ಠ
I really wouldn't, if I were you. Just... look away.
Yeah, okay, I'm pretty much just fucking with you now. Skip down to the next one.
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.
Find it here! (Locally hosted, HTML)
All you need is a stack of blank index cards, some pens, and your imagination.
Like 1,000 Blank White Cards, but more complex, with an aim as a testing system for building your own card game.
A quickstart PDF for Call of Cthulhu. Great for reference.
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."
Masterless Dungeon Crawling: Each player is both an adventurer AND a DM, taking turns telling the story.
Character sheets for just about every tabletop in existence.