Well, there's your many-voted Mario Kart DS, but you'll either find people who use crazy cheats, and then snakers.
There's Metroid Prime Hunters and it's many glitches.
And then one of my favorites, Custom Robo Arena. Play it for the online. Not the story. But you gotta play through the horrid story to unlock more parts.
Worms: Open Warefare 2 is a pretty nice online game.
Tetris DS is online, if that's your cup of tea.
Clubhouse games is online.
Picross is online, but I never got the hang of it.
And then there's pokemans. That's online.
There's a game with the words "Create and Race" in the title. Realistic car-racing game that's online (And believe you me, it's not Mario Kart. You can't floor the gas pedal all the time. LOTS OF BREAKING INVOLVED.)
Tony Hawk's Proving Ground is online.
The Legend of Zelda: Phantom Hourglass is online, but it's online play is nothing like the main story play.
That's all I can think of for now. Ask me about any of those games, and I can tell you more.