If you haven't been to dictionary.com in a while (like me) you will find that it is not your "parent's" dictionary.com. It has been updated with some intriguing new features such as a reference section, translator, and the exciting new (beta) Word Dynamo. This looks like a great place for kids of all ages (college & up as well) to work on their vocabulary skills. I really like that they also have a place for you to build your own vocabulary word lists and then share them with the community or search for other word lists created by others. To set a baseline, you should take the challenge when you first get to the site and then your games are tailored to your level to help you build your brain. For more info, take a tour of what they have to offer.
Word Dynamo is a great place for your students to head after school to review their vocab or work on some new words. You can even explore words by content or user created lists. I thought it would be fun for students to take the "Word Dynamo Challenge" now and then again at the end of the school year to see how much their scores increased (hopefully). You can also use this in your classroom as a transition activity or closure.