Flagon has released his project Net Tic Tac Toe for both PSP and DS. I bet you know what Tic Tac Toe is, so I won't have to explain anything about the gameplay. However, this homebrew lets you play between a PSP and a DS. Currently the IP address is hard-coded into the source, so you can only launch it on two handhelds and wait for it to connect on its own.