SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
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.
Lekh Diagram - Lekh Diagram empowers you to express your idea on iPad and iPhone simply by sketching shapes with...
WebSequenceDiagrams - WebSequenceDiagrams is a web-based application that allows the user to use a proprietary language to define sequence diagram.
Core2D - A multi-platform data driven 2D diagram editor.