It started with a desire to draw diagrams of hierarchical systems with Python. Since this is similar to what we do in CubicWeb with schemas of the data model, I read the code and realized we had that graph submodule in the logilab.common library. This module uses dot from graphviz as a backend to draw the diagrams.
Reading about UML diagrams drawn with GraphViz, I learned about UMLGraph, that uses GNU Pic to draw sequence diagrams. Pic is a language based on groff and the pic2plot tool is part of plotutils (apt-get install plotutils). Here is a tutorial. I have found some Python code wrapping pic2plot available as plugin to wikipad. It is worth noticing that TeX seems to have a nice package for UML sequence diagrams called pgf-umlsd.
If (only) I had time, I would extend pyreverse to draw sequence diagrams and not only class diagrams...