DigiKam is far more fleshed out, however, and full of options and features for the tinkerer. It's difficult to say who digiKam is for, though. The Library tool is exceptional, allowing you to sort by date, tag, facial recognition and even a 'fuzzy search' that tries to match general shapes to an existing image - you're invited to 'sketch' the shape you want to find in a blank box and digiKam will try to find it.
Aside from the slightly variable results from fuzzy searching and face matching, though, digiKam can prove to be a bit intimidating for the very casual user, without containing powerful enough editing tools to please the professionals.
AfterShot Pro, as the name suggests, is a proper tool for proper photographers. It has a sophisticated library manager, a reasonable image editor and arguably the best RAW development tools of any software on any platform. It's also not expensive when compared with the likes of Lightroom or Aperture, although having said that it isn't cheap enough for the faint-hearted.
Darktable is designed purely for handling RAW files, but is better suited - in our opinion - to the enthusiastic amateur photographer than the pro. It's got an incredible number of features, and can do everything that AfterShot Pro does, but it's infused with the spirit of open source and too infinitely tweakable to fit comfortably into a serious workflow. Wrestling every picture into perfection is fine if you only have two or three to fine tune - not if you need to process hundreds of shots in a day.
Gimp, meanwhile, is still overwhelming for the novice despite a recent makeover. However, lack of competition for picture editing functions, like a clone brush and layer control, means that it'll find its way into most photographers' toolboxes, whether you're just brightening party shots taken on your phone or looking to create artwork for high street advertising campaigns.
AfterShot Pro - 5/5
Gimp - 4/5
Shotwell - 4/5
digiKam - 4/5
Darktable - 3/5
Lots of images to look after? Which one excels at it?
Most of us start off managing our image libraries using a standard file manager and some cleverly named folders. Some of us stick with that through our entire careers. But why complicate things when you can double-click on 'Christmas 2012' and Nautilus or Dolphin et al will throw up a page full of browsable thumbnails?
With one notable exception, all of our candidates have some form of library management in their design. Gimp is a pure editor, and wisely eschews trying to do too much.
Library management is at the heart of Shotwell and digiKam, and both do the job well. DigiKam will sort your shots by any combination of folder, date and EXIF or tag you want, although the interface can be a bit cludgy once you move away from the file tree. Shotwell is a little more limited, as it sorts everything by date first - which can be problematic if you don't tag your shots with subject information.
AfterShot Pro, meanwhile, has a very sophisticated library function that allows you to sort photos according to just about any parameter. Unfortunately, it's let down by an import function that is overly complicated, and won't automatically watch folders for updates, so the in-built file browser is better.
That's probably the reason why Darktable's library management reflects the file structure on your hard drive - it's simple and lets the app get on with generating fast, browsable thumbnails.
Fotoxx, again, displays its weirdness here. It should be a library manager, but operates more like a file browser, in which there's no obvious way to go up a level. Odd.