I tried Wordle, but it didn't seem very polished. i.e. kinda like a first try at programming. or maybe it could just use an artist to make it not look like a free app...
for text twist style games there's also Word Whirl, which I think is slightly better. for example it has audible feedback when you press the letters - makes for a much better UI, I say. also multiplayer mode. there's a free version, and a $1.99 one with more features.
there seem to be a bunch of other text twist clones on the iPhone, but I haven't tried them all. still looking for one as good as the original
actually, although it's not free, IMO wurdle (with a "u") is a much much more fun and addictive word game, I've been playing that for a while now. well worth the modest cost. far more polished than either of the above games, in every way.
another fun free word game is Scramboni. you compete against people over the internet, trying to unscramble a word. 1 word each round, and 20 rounds to a game. some tough competition especially at higher levels! you can play on your PC too. only downside is that it takes about 15min to play a complete game, so don't plan anything else once you start!