Gimp, for example, has a large and well established library of plugins, including one that makes it look like PhotoShop. Fotoxx, meanwhile, treats plugins as simply a custom menu command to launch an external editor.
Shotwell and digiKam come with most of the available plugins already installed - and many are geared up for accessing photo sharing sites without leaving their respective environments.
In Darktable nomenclature, every function and set of image sliders is a plugin. It's highly extensible, in that you could add more than the default set of functions, except that there aren't any extra ones to download - as far as we can see. If Darktable can capture a large enough audience these will surely come.
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When AfterShot Pro was known as Bibble, there was an enthusiastic community of plugin developers, both free and commercial. The good news is that these have gone with the project to Corel, and the forums there are full of homemade packages for geotagging, framing and generally messing up or improving pics as you see fit.
AfterShot Pro - 5/5
Gimp - 5/5
Shotwell - 3/5
digiKam - 4/5
Darktable - 3/5
Fotoxx - 1/5
Happy with your editing results? What about the file format?
There are two questions to address here. Firstly, what's the quality of the final photo like? And what can you do with it? Gimp, for example, isn't yet capable of 16-bit colour precision, which is a problem for professional photographers working in print. It also has an annoying insistence on working with its own file format, .xcf.
Almost everything in Gimp 2.8 was an improvement, apart from the decision to remove other file formats from the Save as dialogs and move them into an Export menu. There are logical reasons for this, but it gets in the way of established workflows and introduces two extra layers of dialog boxes just to output a JPG in the format it was opening from.
DigiKam, meanwhile, is the opposite when it comes to file formats and is happy to upload and download from any photo sharing site, too. Some of these online plugins are a little unreliable, and the chances of tags and metadata getting through unscathed are variable.
Shotwell has fewer online plugins, but all the main social sites are covered just fine. For a RAW converter, AfterShot Pro has a surprisingly diverse range of output options. There are no direct plugins for online sites, but you can create everything up to 16-bit TIFFs in terms of quality and output to ready-made web galleries or contact sheets. It means that for many shots, no external editor is required to get the perfect picture from camera to client fast. It's not without quirks, though - the output dialog is over-complicated and offers to add more effects, like sharpening, without a preview.
In the most recent update to AfterShot Pro the developers also addressed its previous biggest flaw: the default colour balance for pictures is not much more natural, and not quite as eye-poppingly 'contrasty' as before. So it's easier to get great quality shots first time.
And that brings us back to our major criticism of Darktable - despite the apparent simplicity of the interface, it's complex to use, which increases the chances of making a mistake. You can't save an image directly after editing it, for example - you make the changes, go back to the thumbnail view, then find Export Selected Images. It'll suit some workflows, but makes it inflexible to use.
As far as image quality goes, however, Darktable is capable of results on a par with AfterShot Pro - if you can master the controls.