darktable

darktable is the Lightroom replacement for people who shoot raw. You point it at a folder of photos, work through exposure, colour and lens corrections, and export finished JPEGs, with the original file left untouched throughout.

It is not a gentle landing. The controls are grouped by how the image is processed rather than by what you are trying to achieve, so the first sessions involve a lot of reading. Once it clicks, the batch tools are quick and the results are as good as anything paid.

The part that hurts when switching is history. Your Lightroom catalogue, collections and past edits do not come across. Only the original photo files do. If your old edits matter, export them as finished images from Lightroom before you leave Windows. If a simpler photo library is what you actually want, look at digiKam instead.