XFCE is one the lightest Desktop environment for unix like operating system.If you are looking for alternate of Unity Desktop which must be lightweight desktop environment so here XFCE is the answer. The XFCE desktop can be installed along with Unity and any other Desktop environment hence there is no requirement to remove any older desktop environment which you are currently using.The installation method for XFCE desktop is very simple.
To Install XFCE Desktop in Ubuntu use the given below command

Tested on: Ubuntu 13.10

sudo apt-get update
sudo apt-get install xubuntu-desktop

After installation there is no need to restart the system you can get the XFCE desktop by log off.
Now logoff from current user and login back once again but click on ubuntu logo which is at right corner at user login box. On clicking the ubuntu logo, menu will appear select the XFCE one.

Project URL : http://www.xfce.org/