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.
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.
Fireworks: vector based, built for websites and web apps. Unfortunately it's from Adobe now too, so you deal with bloat that Macromedia wasn't very fond of.
Most advanced features of Photoshop, are better left to the professional designers. If you are using them, then you probably are wasting too much time prototyping.
Fireworks (or Gimp, or paint.net), or Visio are good enough.