Cambridge Dictionaries Online
Cambridge Online English vocabulary and Idiom and dictionaries.

vocabulary.com
Want to improve your English vocabulary mastery while having fun? Check out the word puzzles on this site.

Illustrated Vocabulary
For an illustrated view of French vocabulary, check out the Illustrated Vocabulary. It's a fun way to learn elementary vocabulary, with plenty of pictures.

The ARTFL Project
Find exactly the right word with Roget’s Thesaurus.

Modern English to Old English Vocabulary
This site contains a list of modern English words with their corresponding old English equivalents.