Some kind of a competitor to Visio, but aimed squarely at software engineers. Produced by http://www.confluent.com.

This product is now freeware, it runs on HP-UX, Solaris and Windows. For Linux users, it can be successfully
run under Wine.

Its homepage is: http://tersesystems.com/code/?overview=vt14

There's also a mirror available at http://freeware4u.com/modules/mydownloads/singlefile.php?lid=141&title=Visual%20Thought

Another place you can get it is http://sdt.web.cern.ch/sdt/VisualThought/
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It is a very capable drawing package, particularly good for diagrams with symbols and attached connections. Has anyone created new symbols/palettes for it?

''Can you describe it a bit more? The documentation at the URL above is sparse and would like to know some more before spending time downloading it. Does it have templates for UML etc?''

Yes, it has "palettes" for everything, including UML, ERD, Flowchart, Booch, HP Fusion, Jacobson, OMT, network and logic circuits.