Word Driven from Headlight Software, Inc. transforms a cherished car trip game into a fun iPhone application. You're given three letters to work with (in the real world, you'd get those letters from passing license plates, but here they are automatically generated by the application) and must produce a word using each letter in the proper sequence.
The underlying dictionary seems pretty robust, handling nearly all the words I threw at it. At each possible completion, the application chimes to let you know you can submit the word as-is. If your word continues on further, you can keep typing, otherwise you can tap to submit.