A while back I ran across, Got Brainy. This user driven community takes vocabulary flashcards to a higher level, by adding an image and voting system. Users can upload their own BrainyPics and the community can vote to determine if the sentence is good or questionable usage of the word. They have a great word list and even contests for users. Below is an image from the site (note the voting at the bottom of the image).
I could definitely see myself using this with middle school students. It might also be fun for us to create our own vocabulary website using a similar model.
What online tools are you using to teach vocabulary?