The Word Chase ($1.99) is downright addicting. Upon first launch (the app is supported on iPhone, iPod Touch and iPad) I was sucked into the gameplay and found the app hard to put down in the middle of a game.
The object of The Word Chase is to make new words by changing one letter at a time. When you start a new game of The Word Chase, you can select the length of the words you want to work with -- from three to five letters -- and choose one of three difficulty levels.
The app will start you off with three random words and seven letters to start making your new words. After you've used all the letters, you can to make new words, or you can draw for a new batch. This continues until you've used all the letters in the round or get stuck. The Word Chase offers a hint function and wild cards that can help you out in this situation.
You'll want to aim for the highest score possible. Each letter has a different point value that goes toward your real score, but you can also score bonus points for making longer words more often, or using your column of letters without drawing more tiles. Placing tiles incorrectly, drawing new tiles before using all your letters, or using your hints will deduct points from your bonus cache, so play wisely to earn the best score.
Really, the only thing I didn't like about The Word Chase is that you can't play forever. There are only seven levels and once you've finished the final stage, your game ends. But trust me, you'll keep going back to try to beat that score.