Howtoforge published a guide about enabling Compiz Fusion on an Ubuntu 11.04 (Natty Narwhal) desktop with the classic GNOME interface

This tutorial shows how you can enable Compiz Fusion on an Ubuntu 11.04 (Natty Narwhal) desktop with the classic GNOME interface, i.e., no Unity (the system must have a 3D-capable graphics card - I'm using an NVIDIA GeForce 8200 here). With Compiz Fusion you can use beautiful 3D effects like wobbly windows or a desktop cube on your desktop.

  Enabling Compiz Fusion On An Ubuntu Classic 11.04 Desktop (GNOME)