Dia

http://projects.gnome.org/dia/

http://live.gnome.org/Dia/Links

http://left.subtree.org/2007/12/05/database-design-with-dia/

http://projects.gnome.org/dia/faq.html

# Q: What format is the .dia file? A: The native Dia file format (extension .dia) is XML compressed with GZip. You can select not to compress the files in the Preferences or in the save dialog.

http://projects.gnome.org/dia/custom-shapes