Emacs Major Mode for Sequence Diagrams 12 years ago
It might be interesting to update this to use http://www.mcternan.me.uk/mscgen/ or the enhanced http://sourceforge.net/projects/msc-generator/ to remove the websequencediagrams.com dependency. I know others have suggested planetuml, but I've always liked the msc-gen stuff for sequence diagrams, a bit more.