WebSequenceDiagrams
LucidChart
draw.io
SequenceDiagram.org
PlantUML
Gleek.io
yEd
Gliffy
Vim Python IDE
Vim Python IDENo features have been listed yet.
Online sequence diagrams from code. Used to be good but now lacks functionality and design. Has interactive diagrams which is cool.
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.
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.
Gleek.io - Using gleek.io you can quickly map out systems using easy-to-understand syntax, without the need to drag and drop. Give gleek.io a spin and leave cumbersome mouse-clicking behind.
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.