Save the Words is a site that is dedicated to "saving" words that have been eliminated from our current dictionaries. The site is dedicated to bringing them back. Use the site to expand your students knowledge of words. Ask them to find a word learn the definition and use it in their writing.
Visuwords is perhaps my favorite! It feels like a living testament to the power of words. Be sure and check that link out!

http://www.visuwords.com/
An online graphical dictionary/thesaurus that seems alive with connections to other words and meanings. Type in the word you are looking for and press enter. Double click on a node to expand the tree or zoom in and out. Hover over a word to get the definition.

http://www.lexipedia.com/-
A visual dictionary that shows the part of speech, synonyms, antonyms and more.

http://www.savethewords.org/
Click on a word as it jumps out at you and learn its definition. Make a promise to use it in your daily language. Use this site to get students thinking about vocabulary in a fun way! Powered by Oxford Dictionaries.

http://lingro.com/
An free online multilingual dictionary.

http://www.merriam-webster.com/
This free online dictionary and thesaurus includes a word of the day and word games.

http://www.wordia.com/
Find videos that help illustrate the meaning of the words. Great site for High School students.

http://blachan.com/shahi/
A visual dictionary that uses Flikr images on this wiki dictionary.

http://www.teachers.ash.org.au/jeather/maths/dictionary.html
Hope these help!
