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.
PabloDraw - PabloDraw is an Ansi/Ascii text and RIPscrip vector graphic art editor/viewer with multi-user capabilities.
WebSequenceDiagrams - WebSequenceDiagrams is a web-based application that allows the user to use a proprietary language to define sequence diagram.
JavE - JavE (Java Ascii Versatile Editor) is a free Ascii Editor.