Block Club is an interesting multiplayer (or vs computer) game that is reminiscent of Tetris in a few ways, but offers gameplay that allows it to stand on it's own. There's not much to the game itself and it's not exactly tearing up the PSP's graphical abilities, but it is somewhat fun and most importantly, it's free! The objective is to place all of your pieces on the board. Here's some simple controls for the demo: O=select piece/place piece - X=unselect piece - L and R=turn piece.
I couldn't find any screenshots of this game online, so I made my own, sorry about the quality.
Click below to download the demo and enjoy!