SiteViz is as its name suggests, a tool for visualising a sitemap.
Integrity Plus, Pro and Scrutiny can export their sitemap data in the form of a .dot file, which contains the information necessary to draw a chart.
Early on, Omnigraffle did a great job of displaying these .dot files but it became expensive. There were other options but nothing free and easy to use. That's where SiteViz came in and its functionality has been built into Scrutiny.
Its default chart didn't look great though. The layout was flawed making the charts less than ideal.
Version 3 contains some vast improvements to that 'bubble tree' theme, along with some improvements to the colouring. Nodes can be coloured according to links in or 'link juice'. (Think liquid flowing through your links). Connections can be coloured according to multiple criteria too. The charts now look much more professional and are much more useful, especially the bubble tree theme. The screenshots you see on this page were made using it.
It will remain a free viewer. You can export the chart as a png image or pdf, but it can't create the .dot sitemap. For that you'll need a website crawler like Integrity Plus or Pro.