SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
Diagramo - Diagramo is an online flowchart software - diagrams (create, edit and share)
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Modelio - Modelio is an open source modeling tool supporting natively UML, BPMN and model exchange with XMI.
WebSequenceDiagrams - WebSequenceDiagrams is a web-based application that allows the user to use a proprietary language to define sequence diagram.
ARIS Express - ARIS Express is based on the proven ARIS methodology and industry standards.