If you like word games like Scrabble or SpellTower, then you'll want to take a look at WordsWorth. According to the developer, WordsWorth landed in the App Store in 2009 and was one of the first word games available for the iPhone. It's very similar to PopCap's Bookworm game, but WordsWorth has a variety of features which makes it that much more enjoyable to play.
The game presents you with a grid of letters that you use to spell words. You get points for spelling words using adjacent letters, and you get more points for longer words. In single-player mode, you have control over word size, grid size and shuffle count. You can also select from the Classic game, a timed mode, and tumble mode which starts off with a few letters and gradually drops them into empty spots. Tumble mode is especially challenging as you have to spell words faster than they're falling, because the game ends when the grid is full.
WordsWorth also has a multiplayer mode, an added benefit not often found in word games. You can choose from local multiplayer, which lets you play against your friend in the next room, a Play With Friends mode that connects you with people you know and a random mode that connects you to a stranger for a quick pickup game. To take full advantage of the multiplayer aspect of the game, you need to sign up for an account with the game's developer, 99Games.
WordsWorth also supports Game Center, so you can save achievements and climb the ranks of the Leaderboard. If you setup your Facebook account with the app, you can share your game highlights with your friends. If you dislike sharing, that's fine, as it's optional. The button to share on Facebook only appears when you reach a milestone or the game ends.
WordsWorth is an excellent game for those who like variety. With all the different single and multiplayer modes, you'll never get tired of spelling words. The app is available for free, but it has ads which I found to be slightly intrusive. I kept getting one for "Sexy Single," which makes the free app definitely not safe for kids. I would recommend spending US$1.99 and purchasing the full version which doesn't have ads.