I've posted before about vocabulary instruction, semantic mapping methods, and Wordie.org. (For those without the time to go back and read three previous posts, here's the summary: if you do decide to do vocabulary instruction, students should choose their own words for the lists and they should use semantic mapping methods to learn the new words - otherwise they're more likely to memorize the words for the test and forget them the next day. Wordie.org is "like Flickr, but without the photos." It's an online tool I've been trying to use, somewhat successfully, for semantic mapping.)
I'd try to say something about this site in relation to all that, but I think it speaks for itself pretty well. I'm also in a planning period right now, but maybe I'll think of something/have more time later today.
Seriously, it will blow your mind.
UPDATE: Okay, so now I do have a few brief points to make about Visuwords.com. In her book Words in the Mind, Jean Atchinson argues that words are organized in our mind in groups by relationships. Aitchinson finds that there are four ways in which words may be associated: co-ordination (words on the same level of detail, such as hot, warm, cool. This group includes antonymns.); collocation (words likely to be found together, like salt water or butterfly net); superordination (an overall term that includes the stimulus word, such as bird for sparrow); and synonymy (words that have a similar meaning, like hungry and starved).
I was using Wordie with my students to create a semantic model of the mind. We would comment on words we wanted to use, linking them to coordinating words, synonyms, antonyms, and superordinating words.
Visuwords makes these connections and displays them graphically. I think this site would be a great way to introduce the concept to students. It might be cool too if students make their links on Wordie during the week (you can learn new words by connecting them to words you already know) and then at the end of the week looking it up on Visuwords to examine some possible definitions maybe they hadn't considered.
If nothing else, it is wicked cool.