draw.io - Online diagramming application
asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
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.
PathVisio - Open-source and free pathway analysis and pathway drawing software. Allows drawing, editing and analysis of biological pathways.