Krendel is a free, universal RPG. It is really an impressive work, well written, with numerous illustrations. The mechanics are also worth a look. I can see the single roll mechanics being adapted to skirmish rules for miniatures as well as for use in RPGs.
40K Unlimited is a set of fan created alternative rule for Warhammer 40K.
Given the general fan anger over the direction Games Workshop has taken its Warhammer Fantasy property, it is no surprise that fan written alternatives have cropped up. One is The Ninth Age, a a set of free wargames rules for playing mass fantasy battles. It looks very well done, and has a large number of army books.
Ghost Manor is a solo adventure based on the Random Solo Adventure system. The author writes:
Ghost Manor is a FREE tiny Random Solo Adventure.
At 37 pages, it takes you on a quick, but fun, journey through a random haunted mansion, filled with ghosts, spirits and even werewolves! Spend a night and gain a fortune...
The book is based on the Random Solo Adventure rule system, though a bit minimized for the free version.
This is released for FREE (or pay what you want - all support is appreciated and makes it possible for us to continue create new gamebooks!) for anyone to download and even spread further to your friends and family.
The League of Ordinary Gamers has a tutorial on making counters for your games.