The World's Greatest GM Screen by Hammerdog Games.
Basically it is a blank GM screen with clear vinyl pockets in which you can stick whatever you want--rules, pictures, etc. And these days, you can find some homemade GM screen insert done by a fan of the game for practically every RPG out there. So you don't have to keep dropping $10-30 every time you switch RPG's.
With the advent of the "landscape" GM screen I kind of wish I had one that went that way, but since I mostly stand while GMing (for lots of reasons) the additional height isn't such a big deal.