This plug-in enables you to use the vast number of existing filters made for Photoshop's Filter Factory as well as create new effects without having to go through the process of learning how to write plug-ins for The Gimp first.
Originally written by Jens Restemeier for Gimp 1.x back in 1997, this plug-in has now been updated for use with Gimp 2.x.
Project homepage is at http://gimpuserfilter.sf.net/
You can find lots of free FF filters that can be imported into User Filter at http://www.photoshop-filters.com/
A programming guide to the original FF for Photoshop can be found at http://mitglied.lycos.de/filterfactory/