Guns'n'Glory is fun... It makes me want the Droid X though.
I'll have to check out HexDefense.
I am a big fan of RoboDefense... specifically the "custom" maps. It's one of the only apps I've paid for and I've been playing it for over a year (I'm above level 190 now).
Glu Mobile has made Android a big focus and has a bunch of games available such as Super K.O. Boxing and Gun Bros.
I don't have an Android phone, but Drop7 is a good puzzle game. I first saw it on Android and had to wait for an iOS version to come out. ĤI don't have a URL handy. The object of the game is to clear number disks from the board, by lining up certain numbers of disks in a row or column. As the game progresses, more push up from the bottom of the screen.
It's a nice change because it doesn't rely on reflexes and reaction time to play. It gives you time to think. I suppose you could keep it running in the background and switch to it when you have time to play.
Gameloft produces most of the "real" games presently available but they don't sell them in the market and their site is not the most reliable re: reinstalls, updates, the like. Still the games are solid so I'd definitely give some of the trials a run but proceed with caution beyond that.