Bike or Die is an action game for Palm OS, based on mountain bike trials riding. It also has a level editor. I like this game for many reasons. First, it is hard. Old-school video game hard. Some of these levels take a lot of practice to beat. Second, because it is a physics-based simulation, there is really a lot of skill involved in controlling the bike. You can learn to do all sorts of crazy tricks with the bike, too: wheelies, stoppies, front and back flips, etc. Third, you can export the "movies" of your best runs to the web site, and watch how other people tackle the levels. This gives the game a social aspect.
Until I played this game, I didn't think the Palm was very suited to action games. This one changed my mind.
If I could add anything to this game it would be the ability to customize the bike. I'd like to be able to try out different tires and suspension settings.