Online sequence diagrams from code. Used to be good but now lacks functionality and design. Has interactive diagrams which is cool.
draw.io - Online diagramming application
SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
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.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Swimlanes.io - Swimlanes.io is a simple free online tool for making sequence diagrams.