Monsters (Probably) Stole My Princess is kinda charming. You're basically just jumping up a vertical shaft and trying to hit as many platforms as you can along the way to take out the boss with a big combo at the end. It's also very very very ungodly short. Most of the replay comes from trying to top the number of platforms you've hit along the way.
Zombie Tycoon is also kinda fun, a RTS-lite game where you direct your hordes of zombies to tear down buildings, etc.