I use different apps depending on what my needs are, everything from the
GIMP 2.6, Krita and RawStudio which are GPL to Canon software (that came with my
DSLR), Photoshop Elements and Corel Photopaint which are proprietary and for
Windows, but run in Linux via Wine or VirtualBox. I have a Bamboo tablet that I
use with Corel Paint Essentials to enhance photos and do some special effects.
It works in Linux, but I cannot get it to be pressure sensitive.

The GIMP takes some getting use to, but it is quite powerful and it is free.
There is a version called GIMPshop which I don't use, but it is made work more
like Photoshop. Rawstudio is good for fast conversions from RAW to JPG when you
want lots of control. Krtia is good and very underrated.

Other tools include Fotoxx, Picassa, digiKam, and Fspot.