Okay so the world has gone compiz crazy, but some of us still use metacity (the default gnome window manager, no fancy wobbles), personally i use metacity over compiz simply because its a bit more stable and for a machine that i work at that’s important, when I’m just checking emails or ircing or whatever compiz is great.
The problem with plain ol’ metacity is two-fold, no transparencies and no gpu acceleration – thus stepping in is composite mode :)
Just to make clear, transparencies are needed not for the nice looks but to get things like avant window navigator and cairo clock working, these need a compositing window manager. using gpu acceleration mearly means that instead of having your cpu waste time and effort drawing your windows, you get your barley used graphics chip to do it.
To turn on metacity compositing you just have to run the following command
gconftool-2 --type=bool --set /apps/metacity/general/compositing_manager True
If all went well you should have nice shadows around your windows.
To turn off metacity compositing you can run the following command
gconftool-2 --type=bool --set /apps/metacity/general/compositing_manager False