GIMP photo retouching
There’s a whole book of it, Klaus Goelker’s illustrated GIMP 2 for Photographers: Image Editing with Open Source Software. Here’s a collection of online GIMP tutorials, articles and sites for retouching and improving digital photographs.
reducing CCD noise (should be always done to reduce noise in images, esp. indoor and low-light shots)
retouching portraits (using the “healing” tool to fix skin tones and blemishes)
post-processing an image with curves, giving a “glowing” effect, softening the light (good for portraits & people shots)
“airbrush a pretty girl” tutorial on smoothing skin, removing blemishes, using blurs and changing colors [archive]
sampling toned images (make your digital images look like great b/w film or with sepia tones)
See the Gimp users group on Flickr.
“Snatchify” a photo image.
A high-pass filter sketch effect (turn a photo of a person or object into a drawing)
Using Perl-O-Tine to slice and cut an image for putting in a web layout.
Use photos to make textured image backgrounds.