I have a great deal of historical fondness for GIMP, dating back to the Spencer Kimball/Peter Mattis Gimp 0.5x. One of their last contributions to GIMP before moving on was to develop GTK as a part of the non-Motif GIMP 0.9x series. Gimp thus acted as a natural rallying point for would-be Gnome developers as they reacted to KDE's choice of the non-open-source-at-the-time QT toolkit. Gimp really has been very important to the open source GUI landscape....
That makes it all the more sad that Gimp development seems to have stalled, relative to other open source imaging packages. Gimp was 8bpp in 1995 and 19 years later Gimp is still 8bpp. I had had a great deal of hope for the GEGL architecture, but these days I get the distinct impression that the project picked an architecture that was beyond the ability of the available developers to complete in a reasonable amount of time. Krita, Paint.NET, and the GIMP-based Cinepaint are all existance proofs that it's possible to implement high color depth image editing in open source in less than 20 years.