Wordnik may be the most robust online dictionary on the web. Wordnik offers definitions from multiple sources, examples of the word used in literature, links to related words, image related to the word, pronunciation recordings, the etymology of words, and real-time uses of a word on Twitter. None of these features on their own are anything new or ground-breaking, but the combination of these features in one place is outstanding.

Applications for Education
Wordnik is an excellent online dictionary to share with students because presents definitions and content in a variety of ways. The variety of content formats on Wordnik means that there is something with which almost every student can connect.

Here are some related resources that may be of interest to you:
Visuwords – Online Graphical Dictionary
Flashcards and Word Searches Using Google Spreadsheets
Ten Spelling Games and Lessons… And a Laugh