NodeBox is an amazing open source visualization tool for Mac OS X. It is basically a scriptable graphics tool, so you can draw things by using a really simple python code editor that's built into the app.

One of my favorite scripts uses the Web library to draw a graph of the hyperlinks between sites. It is rather basic in terms of its spidering capabilities, but I still think it has a heckuva lot of wow-factor.

The following hyperlink graph is actually for mediarevolutions.org (with a depth=3):