CuberXtreme (originally called Cubruis) takes sliding block puzzles to the next level and you can play the flash version online for free.
You control an animated avatar and push the cubes around to solve some pretty challenging puzzles. When cubes that match are pushed together they disappear and when all the cubes are gone you have solved that puzzle.
The beautiful and colorful graphics will draw you in and the challenging and sometimes frustrating puzzles will keep you playing. You start the game with some guided examples of how things work. You then are presented with some small training levels to get you started. As you solve each puzzle another one is unlocked.
One piece of advice -- don't start this if you have something that needs to get done or someplace you need to be soon. Once you start playing this game it is hard to stop.