Man, If i was really good at homebrew I would work non stop to get a WIFI enabled version of this PC game I had played for many many years (5+ off and on).
The name was A.R.C Attack Retrieve Capture. Its a game where you attack the other team...retrieve your flag if they grab it and capture the other teams flag at the same time. Using regular lasers, bouncy lasers, bombs and missles. People got incredibly skilled at this game. It was a free online game open to all players worldwide really. Sierra was the most recent company to buy it...then they said they would stop supporting it. The community had been going for about 10 years. Here is abit of footage. It survives under a new name, Spark, and is unofficial and I haven't played it since. But the ds could handle it and I got as far as programming a homebrew moving the ship sprite with the directional pad and using the touch screen to control the radical (how you aim) I was just getting to adding map collision then I realized I'm a newb and have never programmed before...soo I knew I wouldn't be able to handle all the inbetween.
But I'm sure the DS could handle it and it would be great if the WIFI were included...I would definitely start working on my homebrew again if I had any clue how I would get the rest of it to work (IE. I'm clueless, but if you want to clue me in I'll do what i can)
Also another fun free multiplayer internet game i used to play was Uni-ball, which is still free and available. and I think the DS could handle it as well. Its kinda like a hockey game with space ships
and yeah...if you watch the video...the rap the guy is doing is really about the game...pretty...crazy