Oooh, difficult - I'm not sure I've played any free games.
Having said that, I have played in several homebrew games, which are free! And I love my friend's Star Trek game that we sometimes cameo in when we visit - I'll mention it again when we reach 24th or 25th August - and the Final Fantasy game that is likely to be my focus on 16th.
I have played one free game, thinking about it: my friends' Dockyards game, used the Window system (see the comments section here for a bit more on that game). I like the system; it encourages players to create a character based on how real it is. Rather than focusing on how good - how many ranks or dice or whatever - your character has in a skill, you ask yourself what they're like. Rather than "I have 5 ranks in spot", you'd go for something like "I'm sharp-eyed". And the alternative could be true: 0 ranks in knowledge could change to "I'm poorly educated", or 1 rank in explosives could be "I think I know more about explosives than I do" (the encouragement is to think in adjectives: "good at dancing"; "trainee programmer"; "brilliant chef"). I like it as a way of building a character concept for any game; it's certainly something I'd like my players to consider if I were to run a game.