EnKoDeur-Mixeur (EKD) is an open source software which makes videos, pictures and audio post-production. It can be also used to convert videos in many formats. It is written in python and use the PyQt4 bindings.
Kumaji is an advanced subtitle rendering library aiming for complete cross-platform support, complete and compliant support for all common subtitle formats and, of course, speed. The source code is also intended to be easy to understand and hack at.