Fedora 13 offers a sneak peek at what will become the main user interface of Gnome 3.0. I tried it out on my laptop by installing the gnome-shell package and then running "gnome-shell --replace" on my Gnome desktop.
Note that Gnome-shell needs 3D support to run (along with a bit of extra RAM and processor power than typical Gnome). It makes some use of this support by resizing and shifting windows around as you click. I'll try to explain.
The upper bar is now the only bar. On the left there is a button that says "Activities." If you click that, the windows you have open will zoom into a line (I think OS X does something similar with a key press), and your view of the desktop zooms out to show a menu on the left that offers you recent documents, open programs, and a search bar.
The top bar also shows what program is currently open (with a very tasteful faded icon of the app), the date and time in the center of the bar, your notification icons (in my case, sound, wi-fi, and battery life) and then that user drop down menu where you can set your chat program availability or shut down the computer.
I must say, it's incredibly slick. But not only that, it's simple. There are multiple ways to get to almost anything, and they're all very easy to access. The search bar in particular is a friend I plan on exercising regularly - it seems like a tool that could be very powerful.
Note that it is a bit buggy and may not work in all cases. I'm using the Nouveau nVidia drivers that offer experimental 3D support, so I'm not sure if some of the glitches are because of that or because of Gnome Shell. But that's okay, I knew that this stuff was experimental when I installed it.
In any case, this seems very promising and surprisingly forward thinking. Most interesting of all is its minimal nature - it's simple and easy to understand, and eliminates a lot of clutter that you see in some Gnome desktops. Good stuff; worth checking out if you have a spare minute.
Check out some documentation and pictures at the Gnome Shell website. Some of their pictures are a bit old and ugly, though. This version in Fedora looks pretty nice.