Scrabble Helper. This simple website is a godsend when you're at a loss for words. Simply enter whatever letters you have, and the word you'd like to connect with. Scrabble helper comes up with a whole bunch of suggestions sorted by score.
The site lets you select one of five dictionaries - Scrabble International/US, Lexulous International/US, and Words With Friends. Some people might say this constitutes cheating -- I think if the other side knows you're doing it, it's definitely not cheating. And it doesn't take the challenge out of the game, because implementing Scrabble Helper's suggestions and deciding which words you'd like to connect with still takes a fair bit of thought.
Very handy, though perhaps not one for the Scrabble puritans in the crowd.