RPGaDay (Day 7)
Favourite Free RPG
This is one of those questions where, if you're a game designer, it's really tempting to say one of your own products. So I'd be really tempted to say that my own game, FUBAR, is hands down my favourite free RPG. In the dozens of times I've run it at conventions or in home environments, it's rarely missed a beat...and only once can I remember a bad session with it.
But, if I push this to other people's stuff, then Warrior, Rogue and Mage by Stargazer Games would certainly be near the top of the lost. So too, Ghost Echo by One Seven Design. Both of these games have been really influential on a lot of my design work over the last couple of years.