Spellstacker!
I'm a huge fan of Spellstacker. It's a word game with elements from Scrabble, Boggle and UpWords, and it has a built-in dictionary. Totally addicting!
The guy who developed it has a great sense of humor, and he's incorporated some very entertaining features like "Morale Booster" (which gives you upbeat messages when you have a pitifully low scoring turn) and "Add Meaning to Your Life" (funny messages when you try to place an invalid word on the board). I even enjoyed reading instructions and FAQs on the developer's web site! (spellstacker.com). FUN stuff!
The game is free... until your letter tiles get progressively more dirty; then you'll need to pay 99 cents for Tile Cleaner -- but by the time your tiles are filthy, you're HAPPY to pay because you're enjoying the game so much.
(No relationship with the company -- just a big fan of this game.)