I just tried the game Yet Another Tetris on my BlackBerry 8800 and was pleased with how playable it was. The game is completely free and has some great and challenging playing modes. A new version just came out that speeds things up. If you are into Tetris I would highly recommend the game the only downside is that there is no trackball support.
It does take a few minutes to setup and map the keys but once you get the hang of it the game is quite fun. To get the game working you have to hit the menu button and select setup. Then you select the number keys on your Berry that you wish to map the keys to. You then click the menu again and select OK. There used to be another free BlackBerry Tetris game but it seems to have been removed by Handmark so this is a good replacement.