PSP 5.1 - For some reason, I simply love version 5.1 of Paint Shop Pro.
Pixelmator - there are a few other compositing/retouching applications available that also get high marks. Seems like all of the good ones came out in the last 6 months.
Pixelmator looks nice, but given that I do about 30 minutes of photo editing work every month, at most, $60 is just too much. I'm not saying it's not worth it; I'm just saying it's not worth it to me.
Others I have run across (but have yet to try) are Acorn, Iris, Pixen, Seashore, Naked Light, Chocoflop.