The GIMP has long been praised and reviled as being both a powerful
image editor with features that rival Adobe Photoshop's, but also
being hard to learn. If you're used to Photoshop, you can smooth
your switch to The Gimp by using GIMPshop, a version of the Gimp
modified to mimic Photoshop's interface (as we went to press a
version based on the latest version of The Gimp was not yet
available). A bevy of free Gimp plug-ins add key missing features,
including RAW image support.
Note: This link takes you to the vendor's site, where you
can download the version of the software that's correct for your
Linux distribution. This program is donationware. It is free to
try, but the author accepts and encourages donations towards
further development.
--Scott Spanbauer