Online sequence diagrams from code. Used to be good but now lacks functionality and design. Has interactive diagrams which is cool.
asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
draw.io - Online diagramming application
PabloDraw - PabloDraw is an Ansi/Ascii text and RIPscrip vector graphic art editor/viewer with multi-user capabilities.
SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
JavE - JavE (Java Ascii Versatile Editor) is a free Ascii Editor.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.