Doesn't matter if they are free or cost money, Any fun apps at all, I usually prefer paid ones because they usually are more fun, so list as many apps as you can and Ill award 10+ points to the best answer!!!
Hmm that all depends if your ipod/iphone is jailbroken! Mine is jailbroken and my favorite app is, gameboy4iphone, its a gameboy app where you put gameboy roms on it, great fun brings back all the classics, i think theres also ones for Xbox360, ps3, nintendo ds, all kinds of the emulators.