Word Sense is a neat little service that is one part dictionary and one part thesaurus. When you enter a word into Word Sense it will show you the definition(s) for the word as well as the connections to associated and similar words. You can see any of the definitions of the connected words by simply clicking on them to pop-up a definition.
Applications for Education
What I like about Word Sense in comparison to a typical thesaurus or dictionary is that students can see the connections between the words they started out with and the words they end up using. I also like that students can get clear examples of the words being used in context.