Title: MTB Freeride
No tracks, no invisible walls, and no limits. Just you, your mountain bike, a completely open mountain, and an awesome sense of speed is all you need to have hours of fun in MTB Freeride.
You could follow the other bikers on a vague path to the bottom but why do that when you can explore on your own, race down extreme drops and attempt (and probably fail) to jump huge gaps. The first person perspective adds to the immersion when you're racing through a gauntlet of trees at top speed.
The controls take some getting used to, but then again tumbling down a mountain side in first person is half the fun. The landscapes are beautiful and the mountain is quite large; there are tons of jumps and routes to find.
If you're looking for some fast racing fun, some big air, and an open mountain to explore, MTB Freeride is worth a download. And best of all, you won't have to spend a cent.