AviSynth

AviSynth is a frame server –it feeds uncompressed frames to other applications directly- that can be scripted to automate image processing and editing of videos.

It is not a video editing or special effects workstation. Its strengths lie on allowing the video professional to pre-process videos to resize, crop or remove noise and to conform video sources with simple cuts. Basically you create a script that indicates AviSynth which filters to apply in what order. If you are not comfortable with writing scripts in text form, you could use VirtualDubMod or MeGUI graphic user interfaces to create them.

AviSynth comes very handy when preparing videos for compression in an automated and streamlined fashion. In particular, you can perform noise reduction, resizing and color conversions to ensure that the compressed video looks as best as possible.
It is an open source project that has many followers. Check it out:

http://avisynth.org/mediawiki/Main_Page

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.