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...

-Rob A>