Diagramming software

I've been reminded of AT&T Labs' GraphViz again, most recently by a Drupal developer who's writing code to draw diagrams from Drupal's database. Lately, my organization has been pushing to get reports of user data. The reports we get generated from our sysadmin are mostly raw dumps of data that have some columnar formatting. What we're looking at right now is using log files to auto-generate diagrams that show usage data. Should be fun. If you've done this sort of thing with GraphViz before I'd love to hear about your experience. I've downloaded the Mac OS X package and am learning the languages now.

Other semi-automated diagramming packages (gleaned from the Tulip site).