Cinnamon is a Gnome-Shell desktop fork. It is not strictly a Gnome-2 interface, although the developers aims are laudable - to produce a simpler more traditional desktop interface.

Development seems to be rapid. It is usable and certainly fun to play with with a growing number of extensions to install to extend the base installation.

How to install

for 13.04 & 13.10, 14.10 & later

Cinnamon is now available in the Universe repository

cinnamon

Alternatively - from the command-line:

sudo apt-get install cinnamon-desktop-environment

Since it is in this repository, it is community maintained and thus it is up to the community to update the packages as and when new releases and bug-fixes are released.

For Ubuntu 14.04, there is currently no cinnamon packages in the Universe repository.

Via PPA for 11.10 and later

EDIT: 22/05/2014 - the PPA ppa:gwendal-lebihan-dev/cinnamon-stable has been removed or hidden from public view. I contacted the maintainer and the following is their reply:

The stable PPA is indeed no longer being maintained.

The nightly PPA is being kept for development purposes and should not be used on any sort of production machine (it can and will break at any time).

To be honest, I don't have an alternative to offer Ubuntu users at the moment, apart from switching to a distribution that does support Cinnamon. There are many such distributions out there, and I'm only hoping for someone to (finally) step up on Ubuntu's side to provide proper packages to its users.

EDIT: 25/04/14 - Two alternative PPA's have now been created - both claiming to enable the stable version of Cinnamon for Ubuntu users. I cannot vouch for the veracity of these PPAs

Option 1: 12.04 & 14.04 users only:

sudo add-apt-repository ppa:tsvetko.tsvetkov/cinnamon
sudo apt-get update
sudo apt-get install cinnamon

Option 2: 14.04 users only:

sudo add-apt-repository ppa:lestcape/cinnamon
sudo apt-get update
sudo apt-get install cinnamon

As an alternative you may wish to consider the unstable nightly automatic builds - please note Gwendal's warning above:

sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-nightly
sudo apt-get update
sudo apt-get install cinnamon

Option 3: In addition, you can also install cinnamon 2.2 in Ubuntu 14.04 LTS and derivatives by adding Linux Mint 17 Qiana repository in your system.

Open file /etc/apt/sources.list with nano command:

sudo nano /etc/apt/sources.list

Add the following line at the end of file /etc/apt/sources.list

deb http://packages.linuxmint.com/ qiana main upstream import

Update the repository then install the authentication key

sudo apt-get update
sudo apt-get install linuxmint-keyring

Update the repository again and install Cinnamon 2.2:

sudo apt-get update
sudo apt-get install cinnamon

After installation

Logout and select the Cinnamon session:

Enter you password and you'll see the Cinnamon desktop

... showing the traditional start type menu and applet design from Gnome-2

Extensions/Applets

The Cinnamon desktop uses the same philosophy as Gnome-Shell - functionality can be added to by installing Extensions & Applets.

Cinnamon uses its own Settings tool.

Cinnamon Settings

Cinnamon Applets

Similar to gnome-shell extensions - you can install applets from a dedicated Mint website.

12.04 vs 11.10

I contacted the Ubuntu Cinnamon PPA owner - Gwendal LE BIHAN - 2nd April - this is the reply:

We are currently in the process of finalizing the migration of cinnamon to gnome3.4. The PPA will be updated with precise packages as soon as this is done.

Those packages should be available within a week or 2, and definitely before precise's release.

Note that no more releases of cinnamon will be available for other versions of Ubuntu, since the gnome3.4 libraries are not available on versions of Ubuntu prior to precise.

Precise builds are now available in the PPA and can be installed per the linked answer.

From now on, the Oneiric packages in the PPA will be fixed at v1.4. Thus if you are an Oneiric (11.10) user, your upgrade route should be via the 12.04 distro route, followed by reactivating or adding the PPA.

Alternatively, you may wish to consider one of the other answers to the question.

PPA for 12.04 (nightly build)

Packages for Precise are now available on the nightly PPA :

The stable PPA will take a little more time before it gets updated, any feedback about the nightly packages will be very welcome.

sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-nightly
sudo apt-get update
sudo apt-get install cinnamon

You can download deb files from cinnamon github repository as well

Or you can directly use linux mint repository. Just add to your software sources

deb http://packages.linuxmint.com/ lisa main

and do

sudo apt-get update
sudo apt-get install linuxmint-keyring cinnamon cinnamon-session

Edited on Oct- 25- 2015

To install Cinnamon 2.6 stable

Open terminal and run the following commands:

In Ubuntu 15.04 Vivid

sudo add-apt-repository ppa:moorkai/cinnamon
sudo apt-get update
sudo apt-get install cinnamon

To install on 14.04 , 15.04 , 15.10

sudo add-apt-repository ppa:kranich/cinnamon
sudo apt-get update
sudo apt-get install cinnamon

For 15.10:

After you enable the universe repository, run the following commands:

sudo apt-get update
sudo apt-get install cinnamon-desktop-environment

Current version is 2.6.2

You can install Cinnamon 3.0 on Ubuntu 16.04 LTS via PPA.

Cinnamon 3.0 has been released in April 2016 with improved GTK+ 3.20 support, new accessibility and sound settings and more. Although Cinnamon 3.0 is not available to install on Ubuntu through its official PPA you can install it by adding a community PPA. So use it at your own risk.

Before installing Cinnamon 3.0 on your Ubuntu 16.04, please purge this PPA if you are already using it.

Use the following commands to install Cinnamon 3.0 on Ubuntu 16.04.

sudo add-apt-repository ppa:embrosyn/cinnamon
sudo apt update
sudo apt install cinnamon blueberry

After the installation is finished log out and you can select Cinnamon desktop environment by clicking the icon next to your username.