asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
draw.io - Online diagramming application
Lekh Diagram - Lekh Diagram empowers you to express your idea on iPad and iPhone simply by sketching shapes with...
Nektony Mac Visio Viewer - Nektony Mac Visio Viewer is a fully featured application for Mac OS X operating systems to view MS Visio files.
Core2D - A multi-platform data driven 2D diagram editor.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.