This is one of my favorites! You type in a word, get a definition and a collection of about twelve picture images from Flickr, Google and Yahoo on the sidebar. I like the Google images button best; the images seem to be the most accurate. This is great when you have something technical or nature-related that is really hard to explain in words!
Longman's Dictionary of Contemporary English
One of my go-to resources! I not only use this in reading classes to talk about definitions and parts of speech, but also in grammar and speaking classes to show how a dictionary can be a good resource for using the word with the right preposition or in the right expression. The difference between a dictionary like this and dictionary.com is that it shows the word in its most common contexts, gives lots of example sentences and helps you build related words.
Visual dictionary with pronunciation
Another great site for those hard-to-describe words. You can type in the word, and it will direct you to a category of those words with pictures.