TETRIS DS

[Staplerlock]

I haven't played Splash, but it's meant to be reasonably good and AFAIK it has online play, though I don't know how many people play it any more.

Single player is retarded though.

Thought so. They should try and make a Tetris PSP where you tilt the psp on its side to play, like that one overhead shooter. The screen is almost perfectly shaped.