I teach reading, library, and computer science. One of my computer lessons focuses on having students use the online dictionary and thesaurus. The words are 5th grade appropriate. I allow students to use either the dictionary on the Apple dock or dictionary.com. The dictionary on the dock is great for synonyms but does not offer many antonyms.
The kids really enjoyed locating new words, especially after we discussed how using the online dictionary and thesaurus can really help to improve writing and vocabulary selection.