PixInsight is described as '- Advanced Image Processing. PixInsight is a modular, open-architecture, portable image processing platform available for FreeBSD, Linux, Mac OS X and Windows' and is an app in the Photos & Graphics category. There are three alternatives to PixInsight for Windows, Linux and Mac. The best alternative is DeepSkyStacker, which is free. Other great apps like PixInsight are Siril (Free, Open Source) and IRIS (Free Personal).
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.