To do this, simply install gnome-shell from Synaptic Package Manager or via the Terminal:

Code:
sudo apt-get install gnome-shell
To install the Synaptic Package Manager itself to do it via a graphical user interface, you'll need to install it via the Ubuntu Software Center, as it is not installed by default in Ubuntu 11.10 since for most general purposes the Ubuntu Software Center suffices.

Once installed, you can log out in order to change to the GNOME user interfaces, but to do this on the fly you can execute the following Terminal command:

Code:
gnome-shell --replace
To go back to Unity, use:

Code:
unity --replace
Or the same with:

Code:
compiz --replace
Note: I'd recommend you execute the --replace commands via the Dash as the Terminal window will remain open if executed via it. To do this, press Alt + F2 and execute the relevant --replace command.

If you want a basic, no-frills window management package, there's mutter. Essentially would be a sort of replacement to unity, so to speak. To install:

Code:
sudo apt-get install mutter
To switch to mutter:

Code:
mutter --replace
Again, I recommend you execute the replace command via the Dash (Alt + F2). However, if you switch to mutter in Unity or GNOME 3 there will be no menu bar as such unless you switch to mutter in GNOME Classic environment. More information on mutter here: Ubuntu Manpage: MUTTER - Clutter based compositing GTK2 Window Manager

To uninstall mutter or gnome-shell, use:

Code:
sudo apt-get remove mutter/gnome-shell
Hope this helps.