I'm a bit anti-GIMP due to its unintuitive layout and very un-photoshop-like interface. Some good alternatives I've tried are:
Paint.net:
http://www.getpaint.net/index.html
Pixa:
http://park18.wakwak.com/~pixia/aboutpixia.htm
Pixa is pretty good, but I found it wasn't quite what I was looking for.
There are also a few good webpage-based editors, but I always found that they crashed when you tried to do something too fancy.