asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
PGF and TikZ - PGF/TikZ is a tandem of languages for producing vector graphics from a geometric/algebraic...
JavE - JavE (Java Ascii Versatile Editor) is a free Ascii Editor.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Charaster - Open-source ASCII art editor for browser-based, raster-style editing
Graphviz - Graphviz is open source graph visualization software. It has several main graph layout programs.