PGF and TikZ - PGF/TikZ is a tandem of languages for producing vector graphics from a geometric/algebraic...
PabloDraw - PabloDraw is an Ansi/Ascii text and RIPscrip vector graphic art editor/viewer with multi-user capabilities.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
Graphviz - Graphviz is open source graph visualization software. It has several main graph layout programs.
JavE - JavE (Java Ascii Versatile Editor) is a free Ascii Editor.