I've been playing single player mostly, so, I have no idea what games I can recomend to people when they ask for multiplayer online ones. Anyone got a handy list laying about?

Here a good list ..... http://en.wikipedia.org/wiki/List_of_Nintendo_DS_Wi-Fi_Connection_games

Tetris = pure awesome on the DS.

4 player is the best but it can take time to get 4 people but two player is pretty good too. Just watch out for the crazy good Japanese players and the rare cheater occurrences.

how is it possible to cheat on a DS while playing online?
do they use action replay?

Mario Kart DS is a good game to play online

Jump Ultimate Stars = ultimate online game.

mario&sonic

also mario party ds

Pokemon Diamons and Pearl.
Super Monkey Ball Touch and Roll
...

^good job sherlock on reaching 1000!!

 Mario Kart is quite a good online game to play, slighly under populated though i admit, Metroid is full of cheaters and glitches.