"What free RPG did you enjoy most? Give details."
A few years ago, when I was freshly off the car trip to Santa Fe, had only made a few gaming friends, and really only knew the extent of Call of Cthulhu, D&D 4e, and Pathfinder, I went to Bubonicon in Albuquerque, NM. There, I encountered various authors on self-funded book tours, saw a shitload of steampunk cosplay, and sat at a table for a half hour and played Lady Blackbird.
What I recollect from those halcyon thirty minutes of gaming was a game plot that sounded like the love child of Star Wars Episode IV and Space Balls, and the complete and utter open-endedness of the game. There was nothing plotted out in it, either story or geographically. The included rules specifically state not to plan anything, but to ask your players questions, and let them answer them for you. So I found myself, along with a couple friends, manufacturing an escape from an enemy airship, occasionally rolling dice from a pool mechanic that borders on genius in its approachability, and generally having a great time roleplaying as a varied cast of characters with their own secret motivations and dreams.
My only regret is that the game was so brief (much like this post), but hope springs eternal because the rules are still there, still free, and have options for character advancement, so someday I may find myself in the GM's chair asking my players exactly how they intend to get Lady Blackbird to her wedding in time.