Mario Kart -- it's Mario Kart! Yay! Although we now have six DS owners at the office, we haven't yet had time to pull off a six-player race. I'm pretty sure it will be insane though.
Brain Age -- mildly nerdy, strngely addictive. It's not so much a game, more of a series of mental training exercises. Too bad the speaking portion doesn't easily recognize my voice (and/or my accent)
Pac Pix -- the sort of innovative game we should be expecting from Nintendo. Draw Pac-man using a stylus to catch the Ghosts. The sort of thing you wouldn't be able find on other platforms
Bomberman -- another classic multiplayer game
New Super Mario Bros. -- haven't played the main game much, but has a bunch of cool minigames that are especially fun multiplayer
Mario and Luigi, Partners in Time -- this is the one I'm currently playing regularly. Nice Mario-style adventure-RPG. I'm told it's a sequel of sorts to their GBA adventure, but I wouldn't know. As far as I can tell, the only action in the game is "jump"!