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.
Steve Jackson Games is in the process of developing a new edition of the classic game Illuminati. You can get the new Illuminati playtest files here.
Steve Jackson Games has made the rules for the Classic Car Wars game free for the downloading.
I played the heck out of that game back in the day.