Envisage - A Python Plugin Framework for Extensible Applications
See https://svn.enthought.com/enthought/wiki/EnvisageThree?/Index (https links don't work here)
The goal of the Envisage project is to provide a general framework for building extensible, pluggable applications and in particular (but not limited to!), extensible pluggable applications for scientists and engineers.
Envisage is an example of a PluginArchitecture written in PythonLanguage. It is part of a large set of open source programs called the EnthoughtToolSuite (ETS).