Gimp Paint Studio
I don't think it was mentioned before, but Gimp Paint Studio is a bunch of "artistic" brush presets (Yes gimp supports presets, even though hardly anyone uses them...), mainly useful for tablet users.
The web site is here: http://www.jesusda.com/blog/index.php?id=314 (in spanish - translation would be welcome ;) )
And a nice gallery using it is here: http://ramonmirandavisualart.blogspot.com/
Not necessarily directly useful for mapping, but potentially useful for digital artists...
A quick update that someone has translated the GPS documentation into english:
Looks interesting. Any idea how you go about opening a .7z file?
They've made some updates of late... so just a bump to remind people of a really good place to start improving on gimp's core.
Also, a link to their google code page http://code.google.com/p/gps-gimp-paint-studio/