I have been a fan of Thinkmaps' Visual Thesaurus for a long time (since about 2001 by my reckoning which, in software years is, like, forever, man!!).
I have used their outputs a number of times in my teaching because I think they are fabulous combination of software know-how, visualisation and knowledge of language.
Revisiting the Visual Thesaurus homepage after a long absence (sorry, guys) I find that they have added a lot of ideas and links for teachers.
The basic deal is still the same: you can try it out for a few words, but if you want all the options - and there a more than enough to put a smile on any lexicographer's face - them you need to subscribe to a full version.
Basically, just like a thesaurus, you start with a word for which you want a replacement. The Visual Thesaurus (VT) does not just provide you with semantically related alternatives, it colour codes them for part of speech, and visually represents 'closeness of fit' by varying the distance between the search word and its alternatives. It provides definitions, will pronounce the word for you and gives the type of semantic relationship between words. From here you can find out how it works - or at least how you work it, cos they probably won't let you know their trade secrets.
The really fun part is to then just surf through the thesaurus, by clicking on a new word which becomes the central node in a new network. As you see your original word pushed to the boundary and then fade into the distance you are taken on a semantic journey of related ideas and connections. VT will even do this for you in "Autopilot" mode. Sit and watch the connections fly.
A few examples from the limited sampler will give you an idea of what they offer: