The GIMP is the GNU Image Manipulation
Program. By now it's probably about the best pixel-oriented 2D graphics
package available for UNIX. And best of all, it's free.
Another good thing about it is the relative ease to create plugin's for it, any many are already available. See the plugin-registry for more.
At the moment I only have one plug-in to offer, but more will follow.
Also, make sure to visit the official Gimp User's Manual for more information on the use of my selective blur filter.
The Selective Gaussian Blur filter is similar to
a regular gaussian blur, except that neighbouring pixels that differ more
than the given maximum delta parameter will not be blended with.
This way with the correct parameters, an image can be smoothed out without
losing details. Further down you will find some examples.
I found out that Photoshop 5 has a very similar filter which they
call "Smart blur".
This crappy image on the left I got after digitising a badly taken still on my handycam. The resulting noise is exactly why I wrote this filter, I wanted to get rid of all that noise without losing the details.
(For more info on the device shown visit my Nerdbox pages).
Of course, another application might be to isolate foreground objects on a reasonable smooth background like in the next example.
For more goodies don't forget to stop by my software page. And of course you're also very welcome at my home-page.