SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
draw.io - Online diagramming application
WebSequenceDiagrams - WebSequenceDiagrams is a web-based application that allows the user to use a proprietary language to define sequence diagram.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.