Wordwins.com has a number of tools to help you improve your scrabble skills, including 2 letter words lists, 3 letter word lists and a scrabble solver.
You can choose between TWL (USA, Canada and Thailand) and SOWPODS (everywhere else) dictionaries, and other languages will be added soon.
Isn't this cheating? Well...yes. But hopefully you'll learn something at the same time - and for all you know, your oponent may be doing the same!