I forgot to mention that someone's modified Gimp to work like Photoshop -- the program's called "Gimpshop".
If you want to pay for a program then as hairstream said, there's Photoshop Elements. Or a London company make an app called Pixelmator which is highly regarded.
But if you can do everything you want with iPhoto, then you're already sorted!
For simple stuff (resizing, cropping, rotating, flipping, etc.) I've just found a brilliant online photo-editing site... http://pixenate.com/ ...does exactly what it says on the tin.