GIMP is huge tool that I certainly haven't mastered, but it's so useful to have around.
I used it this week to convert images from obscure formats to common ones. It's a great swiss army knife and runs everywhere.
The GIMP is the only software tool (AFAIK) where selecting content is an undoable operation.
I think this makes sense: selecting things in a complex image is a much more involved operation than in text.