Steve Jackson’s GURPS (Generic Universal Role Playing System) is one of the grand old men of the role playing world. The game provides one of the most realistic, detailed gaming systems ever. Because the combat system is so good, I know of many miniature gamers who use it for skirmish games. Steve Jackson games has released a free “lite” version of the rules as a pdf download.
Tombstone is a free RPG set in the wild west. It’s designed to be rules-light, and has quick play character creation to get you started.
You can get a free downloadable City of Mist RPG Starter Set. City of Mist has an interesting premise. It’s a noir type setting in which you play a character that is the incarnation of mythic and literary characters. You could be King Arthur, for example, or Little Red Riding Hood.
Dead Simple Steampunk is a set of one-page RPG rules for settings in a Victorian science fiction setting. There’s also a one-page setting and one page for equipment.
For those who like to get in on the ground floor, Wildfire Games has an open Beta Test of its new CthulhuTech RPG Core Book.