Kids Wordsmyth is a great visual dictionary that I learned about through Kelly Tenkely's excellent blog, iLearnTechnology. On Kids Wordsmyth students can search for words, hear them pronounced, read definitions, and see illustrations of words. Students can search for words or simply browse through the Kids Wordsmyth online dictionary. The dictionary is arranged in a virtual book format with tabs for each letter of the alphabet.
Applications for Education
Kids Wordsmyth is the type of resource that I like to link to and feature prominently on classroom blogs / websites. When a words of a definition aren't adequate, the picture could help students grasp the definition of a new word.