Online sequence diagrams from code. Used to be good but now lacks functionality and design. Has interactive diagrams which is cool.
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
draw.io - Online diagramming application
SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
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.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
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.