Ion is an X11 WindowManager written by Tuomo Valkonen after implementing PWM to test tabbing and finding that it didn't suffice as a means of increasing the efficiency of use of GUI applications. Ion 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
While Ion isn't big on EyeCandy, it's extremely functional and useful. 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.) After a while (and a little customization) you don't even notice that Ion is there - unlike a normal WindowManager. Although you might think a large screen is required to use Ion comfortably, support for multiple workspaces and implicit tabbing means people have found it works out quite even on a 15" screen at 800x600.