GIMP#, a project to provide a “C# wrapper around the GIMP API” and that “enables users to quickly write new GIMP plug-ins using .NET or Mono” has had some exciting developments recently.
Maurits is working on a GIMP# plugin that will hopefully allow GIMP to use Photoshop actions. AFAIK, these are basically files with calls to Photoshop functions. They are self-contained programs using a Photoshop programming API to perform operations on images. There are a countless number of Photoshop actions for download, free, shareware and commercial, so having access to these plugins would give GIMP a huge boost in terms of what it can do.
There are problems, some filters and functions in Photoshop simply aren’t available in GIMP but he has managed to parse 25 filters from the action files.
Lots of actions map on a GIMP equivalent. I downloaded about 100 action files and created a top 25 of most often used functions. At the top are functions like Brightness/Contrast, Set Selection, Gaussian Blur, etc. etc. The first function without a direct GIMP equivalent is the Chrome filter.