VirtualDub

VirtualDub is one of many very useful open source programs that allows you to quickly convert and process video.

VirtualDub provides you with a framework for basic video editing, compression and filtering. It comes with a few plug-ins for encoding, frame rate conversion and image processing, but there are many third party plug-ins in the open source community that can be added very easily.

http://www.thedeemon.com/VirtualDubFilters/

With this powerful application you can stitch videos together, change audio or add overlays very quickly. This is one tool that video compression professionals must have in their arsenal. You can get it here:

http://www.virtualdub.org/

More technical video professionals can create or adapt their own plug-ins for VirtualDub using its open source filter SDK.

http://www.virtualdub.org/filtersdk.html

As with any open source solution, the main issue is support. However, with VirtualDub there is a very large community of developers and users that you can tap to find answers.