RemoteHQ - Real-time collaboration for distributed teams
asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
Terrastruct - A diagramming tool for software architecture
Core2D - A multi-platform data driven 2D diagram editor.
Diagrams for Mac - A diagram editor for visualizing structures and processes
PathVisio - Open-source and free pathway analysis and pathway drawing software. Allows drawing, editing and analysis of biological pathways.