I work on a different Open Source project that was looking for a BSD-licensed MT-feature-compatible blog package with nice clean extensible code. Unlike some of the other developers here, my goal is not a weblog system that is easy for non-technical people to use, because as far as I'm concerned there are already plenty of those. Personally I am interested in a system that is easy for experienced PHP developers to extend in new ways.
Embarrassingly enough, my personal weblog is not Serendipity -- it's a very old handrolled system that will be a pain to move over. But I contribute to a group weblog that uses Serendipity: