Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Playscii - Full-featured ASCII art and graphic editor and game creation tool
Graphity - Graphity is a diagram editor that can be used to quickly and effectively generate drawings and to...
PabloDraw - PabloDraw is an Ansi/Ascii text and RIPscrip vector graphic art editor/viewer with multi-user capabilities.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
REXPaint - REXPaint is a powerful and user-friendly ASCII art editor.