Puyopuyo is a highly addictive puzzle game from Compile, a Japanese software company. It's a puzzle game where 2 blocks (each one of several colors) come down at once. Place these blocks, as you must connect 4 or more blocks and make chains to get points. One wonderful free version of it, xpuyopuyo resides at http://chaos2.org/.

Of course, the only experience an American will probably have with the game is "Kirby's Avalanche" -- all of the Puyo series are released only in Japan.