About four years ago in this newsgroup, somebody
mentioned that Photoshop 7 was working well
on Linux using Wine.

It took a while, but the Wine team finally got around
to getting Photoshop CS working well in Wine, too;
as of the next release (wine-0.9.45), even details
like pressure sensitive tablets ought to be working.

(Photoshop CS2 also seems usable already, but there's
a bug that you have to hit ^C in the
console window to get past on startup, so it's
not really ready for wide use. And Photoshop CS3
doesn't even install yet, and probably won't for a while.)

If you're interested in helping the Wine team get
recent versions of Photoshop working, please
consider testing Photoshop under current Wine
and reporting the problems you find. See
http://wiki.winehq.org/AdobePhotoshop
for current status and suggestions on how to
give good feedback.
- Dan