http://modeemi.cs.tut.fi/~tuomov/ion/
Ion is an X11 WindowManager that deviates from the dominant overlapping windows paradigm. Instead, it allows you to divide your screen up into a series of frames. You can bind classes of windows to particular frames, and having multiple windows open per frame gives you a series of tabs in the frame titlebar.
Check out a screenshot to get a better idea: 
http://modeemi.cs.tut.fi/tuomov/ion/screenshots/ion-1.jpg
It has excellent keyboard support, and obviates the need for programs like Mozilla to implement their own tabbing. In fact, programs that do their own tabbing are a PITA because you can't use your standard keybindings to manipulate them.
I've set it up so that the main gimp window (the one with the tools) opens in the top right frame, image windows and file open/save dialog boxes always open in the large frame, and all other windows/dialog boxes open in the bottom right frame. This means that the main image window is almost never obscured, and irritating popup windows (like for the !EyeDrop? tool) always open out of the way.
While Ion isn't big on EyeCandy, it's extremely functional and useful, and I've found that it helps me to get on with whatever I'm doing - unlike a normal WindowManager, you don't even notice that Ion is there after a while (and a little customization)
Ion looks a lot like the GUI for NicolasWirth's Oberon operating system. It had simple, non-overlapping windows like that.
It looks like it would only be comfortable to use with a nice big screen. --GlynWebster
Actually, it works out quite well on my 15" screen at 800x600. This is because of its support for multiple workspaces, and the implicit tabbing. I tend to have a collection of fullscreen workspaces for general use, and a bunch of special purpose ones for apps like TheGimp? and XMMS that work best with a specific frame layout. --StephenLewis?
