Update: Keeton sez, "Our website, 1KM1KT, accepts and distributes RPG's in .pdf format as well. The big difference between our site and Steve's is that our downloads are available free of charge."
Update 2: Joshua sez, "If you're interested in RPGs that are on the cutting edge, check out Anvilwerks, my friend Clinton R. Nixon's (yes, that's his real name) role-playing game site. Not only is his game The Shadows of Yesterday a great game, it was published entirely with open source software, as he documents. And all of his games are published under a Creative Commons license"