This past weekend I picked up Lumines for the PSP. This game is pure digital crack, it should come with a warning label.
It is similar to Tetris yet completely different. Square bricks drop from the top of the screen, composed of four smaller pieces in two colors. When you create a 2×2 square of the same color pieces they disappear along with any adjoining 2×2 squares of that color. There’s a slight delay in their evaporation so if you act quickly you can drop more pieces to extend the chain reaction. There are also special pieces that cause all connected pieces of the same color to disappear when used in a 2×2 square.
It has the regular mode where you keep playing until the bricks reach the top of the screen, competitive multi-player, timed games, and a mode where you have to form specific shapes.
Highly addictive. Highly recommended.
Update 4/25/2005: One thing that really makes Lumines more engaging than many Tetris-style games is that the pace varies instead of constantly increasing. You don’t have to do exceptionally well on the fastest levels, just survive long enough to reach the next — which will probably be very slow. The downside to this is that games can be excruciatingly long — I doubled my high score this weekend with an hour-long game.

I agree that Lumines is crack. I have 4 games for my PSP (Lumines, Wipeout Pure, Twisted Metal, and Mercury), and I play Lumines more often than the others by an order of magnitude or more.
I usually describe it to people as Tetris crossed with Bejeweled.
I love how the music/”skin” graphics react to your moves. It adds a lot to the experience, I think, and I’m constantly trying to do better in all the different modes to unlock more skins.
Happy gaming!