PabloDraw - PabloDraw is an Ansi/Ascii text and RIPscrip vector graphic art editor/viewer with multi-user capabilities.
draw.io - Online diagramming application
JavE - JavE (Java Ascii Versatile Editor) is a free Ascii Editor.
Nektony Mac Visio Viewer - Nektony Mac Visio Viewer is a fully featured application for Mac OS X operating systems to view MS Visio files.
asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.