Siril is an astronomical image processing tool, able to convert, pre-process images, help aligning them automatically or manually, stack them and enhance final images.
Siril vs DeepSkyStacker opinions
lots of automated processes self explanatory AND runs on MAC!!!!
Siril is the number one software alternative to DeepSkyStacker
Regim makes some processing steps that are unique to astronomical images a bit easier. These steps are calibration (dark and flat), deblooming, registration, combination, color calibration, gradient removal and some more.
SAOImageDS9 is an astronomical imaging and data visualization application. DS9 supports FITS images and binary tables, multiple frame buffers, region manipulation, and many scale algorithms and colormaps.
Qastrocam is a capture program to do astrophotography. It can control a telescope to do guiding with the images received from a V4L device. It can also control the extended features of a webcam modified to do long exposure (several seconds) captures.