There is really only one option when it comes to editing photographs using Linux: The GIMP (The GNU Image Manipulation Program). This program has a long history, and has evolved to become a highly functional tool with a decent user interface. I tried using The GIMP a few years ago, and gave up quickly -- it was hard to use, and really told me that Linux tools at the time were not up to professional standards.
Today, the story is quite different. Version 2.6 is now out (2.6.2 is the latest version at the time of writing), and it looks to be as capable as tools I've used on Windows such as PaintShop Pro (please note: I'm not a Photoshop user, so I can't compare The GIMP with Photoshop CS3 or CS4).
The GIMP opens up with a tools window on the left-hand-side, and a layers/channels window on the right-hand-side. There is also an image window in the middle, that allows you to open and manipulate images. All three windows are independent -- they can all be positioned independently although all close if the main window is closed. This is a bit different than other image manipulation programs that I've used, and it takes a while to get used to it.
The tool set is available both from the left-hand-side window and from the window menus on the image window. Image adjustments are available from the Colors and Filters menus. The set of adjustments is quite large, and includes the normal set of colour manipulation, hue-saturation, contrast, levels, curves tools, and so on. There is also a good set of layering tools. Essentially, I was able to do any of the standard set of manipulations that I wanted.
I also found a useful plugin for Chromatic Abberation correction (Fix-CA). This tool is very similar to the CA correction in Bibble Pro. It's very easy and intuitive to use, and a welcome addition for my wide-angle shots!
The one downside for The GIMP is that it only handles 8-bits per channel. On rare occasions, such as handling infrared images, I prefer to produce 16-bit per channel images from raw. This allows me more latitude for making the extreme changes needed to make a good B&W image. Unfortunately, I cannot do this with the GIMP.
The new GIMP is a great tool, and in my opinion, with the exception of 16-bit/channel images, gives me all the power I need in an image manipulation tool.
. . . Rob Williams