DeepSkyStacker is a freeware for astrophotographers that simplifies all the pre-processing steps of deep sky pictures.
Registering / Stacking / Simple post-stacking processes to quickly view the final result. / Saving the resulting image to a TIFF or FITS file (16 or 32 bit)
After a shooting night you give all your pictures (light frames, darks frames, offset/bias frames, flat frames) to DeepSkyStacker and you go to bed. The next morning (or is it afternoon?) you can see the result and start post-processing.