Favorite free RPG.
Hmmm...this is a hard one. I've certainly gotten more use from the D20 and Pathfinder SRDs than just about any other gaming resource in the past decade. But I'm in one of my "D20 is the opposite of fun" moods right now. Also, I'm not really sure that "Open Content" is the same as "Free." I've never been the guy to just download the SRD and not buy the books. So, I'm going to avoid open systems. Sorry, OSR dudes.
Because I'm currently in an Urban Fantasy/Horror sort of mood, I going to say Witchcraft from Eden Studios. It's a complete game with art and everything, the entire core rulebook. If you like the genre, but want something different from the World of Darkness, you owe it to yourself to check it out.