Avsp

AvsP is essentially a text editor designed to help create AviSynth scripts.
When you want to compare the video at a different frame, you have to go to the appropriate frame number twice, once for each instance of VirtualDub.

AvsP avoids this ugly mess by having a video preview built right in, no need to deal with juggling windows. And since it's a tabbed text editor, comparing multiple scripts becomes remarkably easy, no running and lining up multiple programs, no jumping to a position multiple times. Furthermore, AvsP offers a unique feature called user sliders, which allows for instant comparison of varying filter strengths, something that is impossible to accomplish with a simple text editor.