A freeware graphics authoring package for most versions of Unix (including Linux). It is at version 4.1 and is available at:

http://bourbon.cs.umd.edu:8001/tgif/index.html

This package is great, and I can personally attest to its ability to help one produce professional-looking drawings and/or presentations.

Some of the great features that Tgif boasts:

One of the disadvantages is no easy way to output gif or jpeg (you must use other translation programs for this). Also, I have not found a Windows version.