http://www.filtermeister.com/

Not freeware per se, but does let you make your own plug-ins for non-distribution for free. You have to pay in order to make your filters distributable.

PowerRetouche (http://www.retouchpro.com/reviews/powerretouche.html) and Goldpanner AcuFilters (http://www.retouchpro.com/reviews/goldpanner/index.html) were made using this utility.

Looks like you need to be a programmer to use this. Not what I would call user friendly but then again very useful to those that unlike me really know what they are doing.
DJ

It seems to have some pretty decent tutorials, but even so, it's way over my head. Could be a neat little toy for others though.

Ed