I am particularly looking at two programs that provide a strong post-processing capability. You use them for different purposes and how they are used is quit different as well. There will be a few more posts on each of these. (What about digiKam? Honestly, I don’t know anyone who uses it, so it didn’t immediately come to mind. For all-around photo-management, digiKam is certainly worthwhile. I won’t speak against it.)
The GNU Image Manipulation Program (GIMP)
Largely, the GIMP is what many people might think of a Photoshop for Linux. Many would strongly disagree–commonly what you hear is this: Gimp is nothing like Photoshop. I think people will agree on this: if you need retouching, layer compositing, text, and pixel-pencil drawing, your choices are pretty likely going to be some version of Photoshop or the GIMP. If you dont want to pay for a copy of Photoshop but want to produce layered screen graphics or high-res graphics for printing, here’s your tool. (And while GIMP has some vector tools for pathings, it is not a vector drawing program–see Inkscape for that).
If you have used Lightroom, (a semi-pro and above level raw photo organizer and post-process workflow program), Darktable shall fill an analagous role. Darktable is has no intentions of being a drawing program. Color control is Darktables primary focus. It’s internals operate on color as 32-bit floating point values, which is mighty accurate. However, this means it wants a 64-bit computer with at least 2 gigs of ram. WIth it you can run through a batch of photos imported from your SD card, pick a few 4- and 5-star photos, isolate that set, apply color correction and “make snapshots” of them to jpg or png images.
I look forward to writing out a few examples comparing and constrasting how GIMP and Darktable are used. Linux and digital photography are getting along quite well these days, and I look forward to helping you get a leg up on these two programs!