The lack of Adobe Photoshop on Linux may be a stumbling block for some people, but if you never intended to spend hundreds of pounds on an art package, there's no problem. There are still plenty of apps that will do many of the jobs you may want to do in photoshop.

GIMP here
The staple in any linux distribution, but available for Windows and Mac OSX as well, this provides you will a lot of power for literally no money. It isn't as comprehensive as photoshop, isn't as well documented and is laid out differently, but you can fix that with: http://www.gimpshop.com/

QTPFSGUI here
For those who like to create HDR images and then tone-map them, this is the app for you. Available in the Ubuntu/Debian repositories, but also available for OSX and Windows, this is a powerful app that will take a handful of jpg's or RAW files and merge them. You can then tone-map them using a variety of different algorithms to create wildly different effects.

RawStudio here
One for linux users only and available in most repositories, or from the site as .deb or opensuse packages and tarballs. This will take the RAW files from your camera and allow you to change various settings - exposure, contrast, warmth, sharpen - and then turn them into jpg's, 8bit or 16bit tiffs or PNG's. It'll also let you crop your RAW files and straighten horizons.

UFRaw here
A variation on the Rawstudio theme, but as far as I can tell, without the batch processing, but it does provide a few more controls for tweaking your RAW files. Also in the repositories, but packaged for a huge number of distro's...

Picasa here
It's a google product, but it's surprisingly useful and will work fine with RAW images, allowing you to apply a number of "one size fits all" tweaks to your images, such as ND Grad, Warmify, Soft focus etc. It's suprisingly useful as a photo manager too, searching your drives and cataloging everything.

ImageJ here
A java based image manipulation and analysis application - it claims to be the world's fastest with the ability to filter a 2048x2048 image in 0.1 seconds!

If all else fails, Photoshop CS2 works great in Wine.

ImageJ is really fast and very easy to programme - http://rsbweb.nih.gov/ij/ It can run on pretty much anything with a JRE.

Good find, added.