I haven’t bought a new game in over half a year but my favs are the brain games Big Brain Academy and Brain Age (love the Sudoku). I’m also a huge fan of the rhythm games and I personally think** Elite Beat Agents** is the best game on the DS. I also adore the Japanese original,** Osu Tatake Ouendan.** And if you’re easily amused, I still really like Nintendogs. I love teaching them tricks with voice commands, however you look a little silly on the plane yelling “Shake Bobo! Shake!!”.
Another vote for Clubhouse Games, and I’m rocking out with Zelda right now, too.
It’s also possible to use the DS for other things-- To run homebrew apps. and games, or as an mp3/video player. Alas, I know for sure that it would be a Bad Idea to provide more info here.
I love my DS.