Best All-purpose Image Editor I've Found
I've used Photoshop (still the Mercedes Benz of image editors if you afford it), the GIMP (another good one, but hard to learn), various thumbnail viewers, and the freebies that come with various OS's, but this is the best all-purpose, intuitive image editor I've ever found: PhotoFiltre.
AND IT'S TOTALLY FREE!
(Though Windows only. Sorry.)