JS Sequence Diagrams – UML Sequence Diagram in SVG from Text
bramp.github.io
bramp.github.io
If you have a good tech architect running a meeting this tool is great to create sequence flows on the fly. It's also got great builtin features like exporting and an API for integration.
Last but not least, it has the Rational Rose theme @haddr mentioned but my personal favorite is boring ol' Blue Modern
And that has a million integrations available, including a pretty good emacs-module[1] ;)
It seems way more streamlined, produces more beautiful results, and probably could be made collaborative as well.
Edit: I made a realtime collaborative plug, here is an example: https://thefiletree.com/espadrine/sequence.sequence
However, the syntax is not as extended, it seems.
Threw together a usermacro for Atlassian Confluence that uses this as a quick and dirty proof of concept: https://gist.github.com/gregmac/fa89a3d86dcb1a008153