Change Unity in ubuntu: 2 Steps

If you like other elements of Ubuntu, but you don't like the user interface (desktop environment or login screen) or if you want to try a different UI, you don't need to change to another distro!

You could install a new desktop environment or configure the new login screen. Also, you can get back to Unity if you like it better, every time you log in you will be able to choose between the installed desktop environments.

Steps:

Step 1. First, you need to install the desktop environment:

For Gnome Shell(gnome3 ):

sudo apt-get install gnome-shell
sudo apt-get install gnome-tweak-tool

Clasic Gnome (gnome2):

sudo apt-get install gnome-panel

KDE:

sudo apt-get install kde-standard
or
sudo apt-get install kubuntu-desktop

(Taken from: http://kde.org/announcements/announce_4.2-beta1/desktop.png)

Light and fast XFCE:

sudo apt-get install xfce4

(Taken from: https://es.wikipedia.org/wiki/Xfce)

A fork of Gnome 2 (MATE):

sudo add-apt-repository “deb http://packages.mate-desktop.org/repo/ubuntu $(lsb_release -cs) main”

sudo apt-get update

sudo apt-get install mate-archive-keyring

sudo apt-get update

sudo apt-get install mate-core
sudo apt-get install mate-desktop-environment

(http://mate-desktop.org/)

Linuxmint (Cinnamon):

sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-stable

sudo apt-get update

sudo apt-get install cinnamon

(Take from: http://commons.wikimedia.org/wiki/File:Linux_Mint_12_with_Cinnamon.png)

Step 2. Once you have installed the desktop environment, look at the icon next to the user name:

I installed Xfce, which is shown, also with the 2 default desktop environments:

Choose your desktop environment before login in. 

Take into account: when you install and set KDE as default you are changing:

A. The desktop environment 

B. The login screen 

If we want to change the log in screen, I hate the one that comes with KDE:

sudo dpkg-reconfigure kdm

A new menu will be prompted inside the terminal, choose which installed log in screen you prefer.
(Guided video dpkg-reconfigure: http://www.youtube.com/watch?v=tKkFVwHGY8I&feature=related)

NOTE:

"Warning: having KDE and Gnome together means you'll have cluttered application menus full of KDE and Gnome applications. There may be other minor integration issues, but none should impact functionality, and you can find help for those on the Ubuntu Forums." (http://www.psychocats.net/ubuntu/kde)

Remember that for any problem that you encounter, you can always execute the ctr+alt+F1 to work in Ubuntu without desktop environment and revert the changes that you made.