Below I have listed some of the most popular software for processing final images. You can visit the websites by clicking the links below to download or read more about them. I have included my opinions below.
I mostly use pixinsight to work on my deep space stacks and then finish up on Affinity. Pixinsight is annoyingly geeky and perversely unuser friendly but you're gonna have to get if you're serious about creating great astrophotos because it is also very powerful. I bought Affinity because it was WAY cheaper than photoshop and I think its great. I particularly like its ability to blend layers using a tool called blend ranges which is basically a quick way of using lumincance mask to combine images. (this tool is perfect for combining the bright parts of sharp but noisy lucky imaging stacks with the darker parts of the slightly more blurry but less noisy regular imaged stacks).
Starnet is FREE and something everyone should use. By removing the stars from your image you can stretch and grade the nebula any way you like without fear of ruining the stars. Its brill and I use it on every image I process. When you're happy with your grade you simply pop the stars back ontop in affinity or photoshop using the 'lightness blend mode'. There is a plugin version for pixinsight too.
IMPPG is in the best tool I've found for processing solar images. It has very powerful animation options too... perfect for catching that erupting solar flare.