First off, I'm not one of those weirdos that thinks cell phone gaming is the way of the portable future, because that's stupid =P I am adding to the problem though as I've gotten several cheap/free games to mess around with. So if anything it may be useful for someone looking for a good time waster.
Number 1 on the list is anything by kairosoft, all of their sim games are awesome. Game Dev Story easily the best, but Mega Mall and Gran Prix are also good. Their town sim type ones I don't like as much, but Edo Town is pretty fun, and the newest one ATM.
Baseball Superstars 2 or 2011 are both free and lots of fun despite my sucking at them and not liking sports games lol. On that note, Super Stickman Golf is also cool and free.
For the more expensive ones, Chaos Rings (both of them) are good if you want an rpg type. I can name a buncha other games depending if people are even interested or not, heh. I've played prolly far too many games on this thing =P