Eighteen is a variation of classic 15 Puzzle game. Not only does Eighteen has 18 tiles to move (vs. 15 in classic Fifteen puzzle) but also Eighteen uses hexagonal board making the game more challenging.
If 18 tiles is not challenging enough for you, you could always increase the size of the board and number of tiles to move, or the other way around, if 18 tiles is too much for you, make the board smaller. All in all the game is not just one but three puzzles (and even 4 on the iPad): "6 Puzzle", "18 Puzzle" and "36 Puzzle".
Eighteen is integrated with Facebook. Once Facebook integration is activated the time it took you to solve the puzzle is posted to your Facebook wall so you could brag how smart you are and challenge your Facebook friends.
Enjoy.