There are numerous, with upsides and downsides:
- Capture One Pro (Paid): This is the most complete alternative in my opinion. It can handle ingestion, taxonomy and editing. Ingestion and taxonomy are adequate but not fantastic. Editing is superior to Lightroom in my opinion. It's a little more complicated than Lightroom but I usually produce better results in a shorter amount of time. Also the processing quality in Capture One Pro is pretty consistently better than Lightroom in my experience.
- Photo Mechanic (Paid): This is solely for ingestion and to a limited degree, taxonomy. It has zero functionality for editing but plenty for moving photos around, copying them from places and to places etc. I often see it recommended by professionals as the fastest way to pull in and pick photos from a shoot.
- DxO Optics Pro (Paid): Zero taxonomy or ingestion capability. This is solely an editing tool. Its main target is superior optical correction and processing quality. I find it delivers on both of these but I also find its "magic" features like "smart lighting" to get in the way. Capture One Pro usually produces better results for me. One thing DxO does to fantastically though is noise reduction. Whatever PRIME is, it does a damn good job. On the other hand, DxO seems to have issues with my Sony A7II where the colour is consistently wrong (colours always have a red offset). I haven't noticed this with other cameras though.
- Polarr (Paid, with a free online version): Solely for editing. It's very much like Snapseed (an Android/iOS app) on the desktop. Interestingly, their desktop app appears to be a repackaged version of their web app (https://www.polarr.co/). I find it to be very high quality and very easy to use but I really prefer having my editor integrated with my library and keep coming back to Capture One Pro. If you're editing things separately, I think Polarr is a fantastic choice. I'd actually rate it higher than DxO Optics Pro (based on the output I've had from it).
- Darktable (FOSS): Does ingestion, taxonomy and editing but doesn't really any of them well. The UI is confusing and (at least to me) unappealing, the editing tools are limited and the taxonomy, while more present, is too complicated to figure out. I've tried to like Darktable but I just can't.
- RawTherapee (FOSS): Zero taxonomy but all the knobs and sliders you'd want when it comes to editing. They've put _a lot_ of effort into their image processing and it's high quality and extremely customisable. You can fiddle with demosaicing algorithms and the like all you want. Hell, you can even implement one yourself with little effort (I wrote one that's currently sitting in a PR somewhere). Same issue with this that I had with Polarr though, I like having my library and my photos integrated.
- Nik Collection (Paid): A product that was acquired by Google and still appears to be developed. They have a bunch of individual tools tailored to particular kinds of output that seem to produce high quality results but should really be integrated into a single package. No taxonomy or ingestion, pure editing.