LXDE, Lighweight X11 Desktop Environment, another fast and lightweight windows manager. I prefer use this than XFCE :p
Here is some steps to install it on Ubuntu:
1. Add the repo line for lxde
$ sudo nano /etc/apt/sources.list
copy this following repo line:
deb http://ppa.launchpad.net/lxde/ubuntu hardy main
deb-src http://ppa.launchpad.net/lxde/ubuntu hardy main
*for another release of Ubuntu, just change the hardy to yours
2. Update and Install LXDE
$ sudo apt-get update && sudo aptitude install lxde
3. Logout your current user and select the LXDE on session selector
my LXDE screenshot, on PvJ-OS :p
Finish . .:p


[...] thank’s for : abz [...]
I tied it now at the 31th of March 2009, at 8.25 PM and could not install it. Is their server down ?
i think no. I can access the PPA, last update (via sudo apt-get update) : 1 April 12:53 PM, +7 GMT.
Thanks