Dictionary
The New Oxford American Dictionary link can be found at the top of the main page.
When you click on "Dictionary", you'll find "Vocabulary Challenge" words, and highlighted charts and tables.
The running list of words along the left is a nice feature, so you can see the words above and below the word you are looking for, just like in a real dictionary.
If you are not sure how to spell
a word, it gives you a list of alternate terms.