If you have a device that's capable of running homebrew, like an R4 or M3 cart, you can run homebrew applications on your DS. There's one called Beup Live which is an MSN Messenger application, so if you have a homebrew cart you should try downloading it. But if you use a Nintendo Wifi Connector to get your DS online, it won't work with homebrew apps, you'll need a normal wireless access point (it's possible to set up the Nintendo connector as a regular access point using Buffalo drivers, but even then homebrew might not work properly with it - I've never yet managed to get a homebrew application online using mine).