There are different applications/games for online chatting over Wi-Fi:
GAMES:
Diary Girl
Pokemon Diamond/Pearl
Metroid Prime Hunters
Advance Wars: Dual Strike
APPLICATIONS:
Wi-Fi Chat (by Infantile Paralyzer)
(Don't know others?...)
I recommend downloading the Wi-Fi chat, because it is better in my opinion.
You can draw pictures, or whatever, and talk privately in up to 1000 different chat rooms.
The games are an added bonus, I guess.