The Visual Thesaurus from Plumb Design is an online program, written in the Java programming language, that runs inside your Web browser. Most Java applications suck. The Visual Thesaurus, however, is the greatest Java app I have ever seen. It makes the English language's glorious complexity into a floating, blooming, living thing, as I sometimes imagine it in my head.
Once it loads, type in a simple word like "happy" and watch the florettes of related words appear like a crystal growing. Click on them and watch the shapes change. It makes me happy just to look.
I guess I'm an editor for a good reason. Thanks to Martin Townsend on the Editors' Association of Canada e-mail list for the link.
P.S. I can't seem to view the Visual Thesaurus using Internet Explorer on Windows, but the Mozilla/Netscape browser family seems to work fine on both Windows and Mac, so try that if you're having trouble.