Dia
http://projects.gnome.org/dia/
http://live.gnome.org/Dia/Links
http://left.subtree.org/2007/12/05/database-design-with-dia/
FAQ
Format
# 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.