Unity is the default desktop environment for Ubuntu. But sometimes it is a little heavy for some users like me.
Thierry has recommended LXDE, a lightweight/free desktop environment for Unix-like platform.
With LXDE, the system runs more fluent than before.
To install LXDE, you have just type, and relogin :
sudo apt-get install lxde
But I don’t really like its design, I prefer Lubuntu than it which is also a lightweight desktop environment.
To install it, it won’t gonna be so difficult :
sudo apt-get install lunbuntu-desktop
You can find a very good article for installing these environments.