Darktable has been around in the open source community forever, it’s been well developed and kept alive by an enthusiastic bunch but the updates are a little slow in coming. Cameras support is fairly limited too which you can check out if yours is supported here.
One of the great features of Darktable (other than being free) is that it does the basics very well, by that I mean non-destructive editing throughout the complete workflow. Fully color managed, supporting automatic display profile detection on most systems, including built-in ICC profile support for sRGB, Adobe RGB, XYZ and linear RGB color spaces. Darktable does have a catalog system for image management of sorts where you can search your image collections by tags, image rating (stars), color labels and many more, use flexible database queries on all metadata of your images; again it’s a little clunky in design but the function is certainly there.
It also supports tethering but don’t expect Capture One type performance. It has the basic curve, colour and all the usual aspects you would expect from a Lightroom replacement at a minimum.
Darktable is available on Linux, Windows and Mac