GNU Image Manipulation Program (GIMP)
Surprised no-one has mentioned this yet... The GIMP has a great reputation among the Open Source community and is actually a highly advanced graphics editor comparable to Adobe Photoshop in some areas (indeed some people have said it is better).
Of course, like most Open Source software, this is completely free, so give it a try :D
I pretty much only use Open Source software here at home (with the exception of windows on two of my machines) due to the expense of software like Adobe Photoshop and M$ Office.
Anyway, download it here
Re: GNU Image Manipulation Program (GIMP)
Completely agree with you on that score :)
I've been using linux at home exclusively for a whole month now and not touched a windows box at all. I've been following a few of the Linux Format GIMP tutorials to try and get to grips with it; and like anything else - once you get used to it it is farily easy :)