I've written a couple of free iPhone apps - Words, which you can read about on this page, and Airports.
Words is a free app designed to assist you when solving Scrabble and WordScraper types games, crossword puzzles and anagrams.
The interface is clean an intuitive. Simply enter the letters you have in your rack, or those you already know from the crossword and Words will provide you with a list of possible words.
If you enter "A?E", Words will return all words beginning with A, ending with E with a single letter in the middle.
Searching for "A?E" will return all words which contain an A and/or an E and/or any other letter. Words currently uses the SOWPODS dictionary.
Airports can be downloaded now from the iTunes App Store.
Update: Words has now been updated for iOS 5.