[mojuba]

There's GIMP, but I don't see a lot of enthusiasm on the Net about it. I think I'll try to find something small and simple that suits preliminary web site modeling and is capable of producing web formats - png, jpeg and gif - out of the slices of the basic graphical model. In fact GIMP may be OK for doing just that.

[DougBTX]

For prototyping, you might want to try something more focused on vector drawings than just pixels, i.e. Illustrator vs. Photoshop. I've not found any great open source vector image editors yet though. In the market for one though.

[bct]

Inkscape's the big one.