Programming plugins for Blender's sequencer is surprisingly easy. What follows is a list of the plugins I've created.

Unsharp mask
Apply an unsharp mask to one strip. Optionally it can be set to only sharpen on the x axis (horizontally), which provides acceptable quality and is about twice as fast.
Files:
Source codeunsharp.c
Linux amd64 binaryunsharp.Linux64.so

Invert colors
Invert one strip one color channel at a time. If all are selected it looks like a film negative.
Files:
Source codeinvert.c
Linux amd64 binaryinvert.Linux64.so

Letterboxer
Dynamically crops one strip to letterbox it to different aspect ratios. The x offset and y offset don't do anything yet.
Files:
Source codeletterboxer.c
Linux amd64 binaryletterboxer.Linux64.so