I know GIMP (GNU Image Manipulation Program) is the software that is very popular since it comes packaged with many GNU/Linux distributions.  However, GIMP has never really done it for me.  As a user of both Photoshop and Ulead PI, I could not easily transition to GIMP.  I have now found GIMPshop, which is designed to have more of a look and feel of Photoshop.  I just started using it and am going to do a full review on is here later after getting more time in on GimpShop.

Check it out here
http://freshmeat.net/projects/gimpshop/?branch_id=57360&release_id=192296
and here
http://freshmeat.net/redir/gimpshop/57360/url_mirror/www.gimpshop.com