AviSynth is an extremely powerful frameserver for Windows that allows using VirtualDub and TMPGEnc plugins as well as tons of its own plugins to be added to the frameserving process.

AviSynth even beats its "big brother", VirtualDub, in terms of flexibility when speaking of frameserving. Possible uses of frameserving are endless -- cropping, resizing, adding permanent subtitles, changing formats to .avs frameserver files, etc before feeding the data to other tools, such as video encoders like TMPGEnc. Also, if you need to add a functionality to "convert" .avs files to pseudo-AVIs for tools that don't support AviSynth's own frameserving files, you can do this by using a software called Link2.

Obviously the ultimate flexibility AviSynth has, comes with a price -- program doesn't have any kind of graphical userinterface, but instead all of its settings are saved within small textfiles that AviSynth then converts on-the-fly to frameserver files. One of the excellent examples of AviSynth in action can be seen in DVD2SVCD bundle which essentially controls virtually all of its background processing by using AviSynth. But if you're not scared of learning new stuff and want to know exactly what is happening to your video and in which order, AviSynth is THE software for you.

Program is released as an open source software and its sourcecode is freely available from its homepage.