Seen on /., GREYCstoration is a library and command line tool for image processing. It looks fantastic for removing noise from images, but also does some nice stuff like removing JPEG-compression artefacts, and scaling up smaller images without the normal levels of distortion. Apparently this library is used in DigiKam and Krita.

Another tool that sounds interesting is Resynthesizer, a plugin for GIMP that can help to create tileable textures and remove unwanted objects from an image.