Starship Defense. It'll own your soul.
5 levels in, and my soul is still intact...
Starship Defense is still a good recommendation though. Fun tower defense-style game that has great (simple) aesthetics.
Other games I've picked up and really enjoyed:
1.
Donkey Kong (GB Virtual Console) - Always wanted this game back in the day, but never got around to tracking down a copy. For the price it's a fun romp. A bit too easy so far, going into the sixth level.
2.
PiCTOBiTS: Art Style - Worth buying just for the awesome chip tunes soundtrack. Not a terribly difficult puzzle game, but lots of room to improve on high scores and fun just to play through.
3.
Pop Island Paperfield - At $2 my expectations weren't very high. Probably a good thing because this frantic game isn't very deep. But the fun multiplayer options using Download Play to compete against anyone with a DS/DSi/3DS system really gives the game legs if you have some gaming friends.
So four games for under $20 to hold me over until the Ambassador program kicks in. Not bad!