If you like your puzzle games to be infuriatingly challenging then SoLiD SHiFT should be right up your street.
The object of SoLiD SHiFT is to clear the board by touching two or more tiles of the same color. The tiles that remain after you've removed these will then shift down or move right. As a result, you need to be very shrewd in the choices you make when removing the tiles.
SoLiD SHiFT might not be the most advanced, graphically-rich game in history, but it certainly provides hours of addictive amusement. Its gameplay is on a par with classics such as Tetris or Columns.
Unfortunately, SoLiD SHiFT doesn't have a huge amount of different options or game modes. There are no difficulty levels or online high scores tables, and this means it's not one you'll still be playing in a year's time.
That said, SoLiD SHiFT is definitely great for a few long bus or train journeys.