Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
APT (Astro Photography Tool) - APT stands for "AstroPhotography Tool" and it is like Swiss army knife for your astro imaging sessions.
GIMP - GIMP is a multiplatform photo manipulation tool.
RegiStax - Photography, Astronomy, Image, and Astrophotography
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.
Nebulosity - Nebulosity is designed to be a powerful, but simple to use capture and processing application for Windows and Mac.