PixInsight is a modular, open-architecture, portable image processing platform available for FreeBSD, Linux, Mac OS X and Windows. 

Website: PixInsight — Pleiades Astrophoto

Tutorials and Help:

• some but no all processes and functions are accompanied by internal documentation that can be opened by clicking on the Process Explorer and then on the process icon or by clicking on the documentation icon at the right lower edge of the currently open process window
• In September 2022 PixInsight announced a new official video tutorial "Introduction to PixInsight" on YouTube available here: Introduction to PixInsight - YouTube
• There are some tutorials available from PixInsight, please visit: PixInsight — Software Tutorials or PixInsight — Resources
• Very good (unofficial) reference guide: PixInsight 1.5 - The Officially Unofficial Reference Guide (deepskycolors.com) the actual version of PixIsight is V1.8) 
• There are many YouTube videos available on the net, I recommend this one for absolute beginners: https://www.youtube.com/watch?v=HIXJJqew6rQ. The chapters below are more or less based on this tutorial.

PixInsight Image Processing Overview

Created with the Personal Edition of HelpNDoc: Easily convert your WinHelp HLP help files to CHM with HelpNDoc's step-by-step guide