I just found this awesome game called Red Remover. The name says it all, really: you need to get rid of the red blocks. But the challenge is that you also need to keep all the green blocks, but they can get knocked off the board by the other colors. Then, after the first ten levels there is an added twist that really throws a wrench into things. It's not enough to just finish the levels, you also have to get par and complete the bonus mode to unlock the extra levels. This is really quite addictive.