I'm a HUGE fan of Photoshop, but when I have lot of programs open and am running low on memory, it's sometimes tough to justify the long startup time to do a quick edit or convert something to JPG.
I stumbled upon Artweaver as a freeware equivalent of Photoshop. It doesn't have everything PS has, but it has enough to do the job - quick crops, resizing, conversion between formats, and it's free. Wins all around.
The nicest part of the program is that it's interface is a very close match to PS, so it's simple to transition from one to the other. The menu choices were all in the same place.
It reads all sorts of file formats (including PSD), it handles layers and offers a plug-in architecture.
If you're looking for a quick and powerful editor for when PS is overkill, you could do a lot worse.