Snappy Words, my favorite, gives students access to other words and their definitions quicker than thumbing through a thesaurus.
Visuwords is also an online graphical dictionary. Students can look up word definitions and discover their associations with other words and concepts. Visuwords allows you to learn how words associate and create diagrams that are suggestive of a neural net. In the process of art creation these tools can be really helpful.
Lexipedia and Wordnik are similar tools with a slightly different look.
All of these tools allow you to enter words into a search box to look them up or double-click nodules to expand the tree or search.