LightDM is the replacement for the GDM login on Ubuntu 11.10. LightDM is a cross-desktop display manager that aims to be the standard display manager for the X.org X server. The crowd greeter will show you creatures walking around and when you click on a creature (each user has a creature) you can enter the password and before you login you will see the other creatures drop dead.
To install the crowd greeter open a terminal window (press Ctrl+Alt+T) and copy+paste the following lines:
sudo add-apt-repository ppa:lightdm-team/crowd-greeter
sudo apt-get update
sudo apt-get install crowd-greeter
When installed you need to edit the lightdm.conf file to do this copy+paste the following line in your terminal window:
sudo gedit /etc/lightdm/lightdm.conf
Now you need to replace unity-greeter with crowd-greeter. When done save and close the lightdm.conf file and reboot your computer.
If for some reason you want to have the unity greeter back open the lightdm.conf file and replace crowd-greeter with unity-greeter then save the file and reboot your computer.