DeepSkyStacker is a freeware for astrophotographers that simplifies all the pre-processing steps of deep sky pictures. It is a free and open source astrophotography processing application that's advanced, portable and light on system resources.
Here are some screenshots to illustrate the features and user interface of this application:
Praat
Able Fax Tif View
Neat Image
AKVIS Enhancer
AKVIS Retoucher