Darktable: https://www.darktable.org/
I've tried it. It's an extremely powerful alternative to Lightroom. It was my tool of choice for the last year or more. But it really is the epitome of what people complain about with open source. It feels like a bunch of features just thrown at a wall with no regard for UX. Even its own loyal users will tell you how ridiculous it can be that there are multiple seemingly redundant ways to do the same thing.

And it provides very little guidance on how or why to use the different options.
Ironically, that post is from a thread about...
Rapid Raw: https://www.getrapidraw.com/
I've just recently started trying this out. It's far too early to give any real conclusions yet. I think I like its basic editing feel a lot more than Darktable, but it's still far too early for me to say. I haven't even finished going through the first project I decided to edit with it.
And to be honest, part of the reason I'm looking at these at all isn't even about open source. It's that I have become dissatisfied with Lightroom. Specifically, with its file management aspects. If I could just have Lightroom 🏴☠️ with file management that didn't feel like a pain in the arse every time I touch it, I'd be happy.
Photo manipulation by Gimp: https://www.gimp.org/
I'm mostly very happy with the latest version of it. It seems to have a really great UX now. It finally feels like an adequate replacement to Photoshop in many ways, which it certainly did not prior to the recent-ish major redesign.
Except that it is insanely resource-intensive. For some reason a fairly simple project consisting of nine 25-ish MB jpegs and basic layer masks ended up for me as 1.5 GB compressed, and I think it may have been 5.7 GB uncompressed, based on what its UI was telling me. This made it painfully slow, in addition to the ridiculous amount of disk space it took up.
And it still unnecessarily fails to have sensible defaults, as I experienced in this thread, though thankfully the wonderfully friendly @oeuf@slrpnk.net was able explain how to fix its broken settings.