GREYC's Magic Image Converter (G'MIC) is a image retouching tool which has been made available as a GIMP plugin. G'MIC comes with a large number of pre-defined image filters and effects (more than 190!) and works on Windows, Linux and Mac OSX. More info and screenshots can be found @ G'MIC homepage.
Update: a new G'MIC version has been released. Ubuntu .deb files as well as source files are available to download @ Sourceforge.
Install GMIC in Ubuntu: download the gmic .deb (either the i386 or amd64 package) and install it, then also download the gmic_gimp.zip package (32bit or 64bit), extract it and place it into the ~/.gimp-2.6/plug-ins/ folder.
Once installed, start GIMP, import an image and go to Filters > G'MIC (G'MIC won't show up if no image has been imported).
Important: I've only tested G'MIC with Gimp 2.6.10 (stable) so if you're using GIMP 2.7.x (still in development), but it should work with any GIMP version (let us know if you've tried it on an unstable GIMP version in the comments!)! Install GIMP 2.6.10 in Ubuntu 9.10 and 10.04.
See also:
- GIMP Plugin Registry 3.5.1 Gets ~129 New Plugins
- Gimp Paint Studio [Gimp Optimized for Drawing and Painting]
Thanks to LFFL for the heads up on G'MIC!
