Hmm.
Castlevainia SOTN is a MUST HAVE. At 800 points, and no less than 10hrs of solid, great gameplay, it's my #1 game.
Geometry Wars is also a ultra-addictive game at 400 points. Very good. Obviously it's the must-have title in terms of downloads, as it's the most downloaded XBLA game in history thus far.
Uno and Texas Hold 'Em, as mentioned. are great online games, if you like those sorts of games. They're also cheap as far as I know.
Settlers of Catan, Carassanone, and Band of Bugs are great strategy games. BoB was alot of fun for me, and has a good online component.
Puzzle Fighter, from what I've heard, is insanely good as well (and was a $50 to boot). High reviews have been given for the game.
The same can be said for Bomberman as well.
Outside of those, just download trials for nearly every game. They're almost all around 50mb, so the DLs are quick.
Back from the dead, I'm afraid.
