In our experience, RawTherapee is one of the best free Lightroom alternatives you can find - so long as you're mostly focused on RAW image editing. It's an open-source RAW image-processing app ...

There's a free tier of Polarr ... But this app remains the best alternative to mobile Lightroom if you want simple, stylish edits. You won't find professional grade tools here.

There is no shortage of Photoshop and Lightroom alternatives out there ... which is a browser-based wonder that is completely free to use. If you are a Mac user, give Pixelmator a try.