KDE SC: An injection of Plasma
The classic alternative to GNOME-based desktops is, of course, the KDE Software Collection (KDE SC). While Kubuntu already provides a ready-made distribution, based on Ubuntu 12.04 LTS, and includes a fully configured KDE desktop, some users might have installed the regular version of Ubuntu to try Unity and changed their minds. Or they might just want to try something new. Luckily, all of Kubuntu's packages are included in the regular Ubuntu repositories. This means that installing KDE SC on Ubuntu is trivial.
To get a fully functional install of the KDE desktop and its applications, open the Software Centre and search for "Kubuntu Plasma Desktop". The package you are looking for is called kubuntu-desktop and will install the KDE desktop itself as well as all other needed packages. During the installation, the system will ask the user to configure a display manager. This is the part of the system that lets users log in and then starts the desktop environment. KDE comes with its own display manager called KDM, but we recommend selecting Ubuntu's own lightdm since this is what comes with the distribution by default. If users prefer KDM, they can switch to that, but for the purpose of this article, we will assume users have selected lightdm here. After a reboot, "KDE Plasma Workspace" should show up in the sessions menu available from the Ubuntu icon at the login screen.
It is worth mentioning that KDE SC has by far the most dependencies of all of the desktop environments mentioned here. This means that its installation will take quite a bit longer than that of any of the other alternatives. But this also means that it comes with a lot of new software to explore.
Xfce: Lightness of form
Like Cinnamon, the Xfce desktop environment has been hailed as a refuge for users who do not like the changes introduced in Unity and GNOME Shell and who prefer GTK+ applications to KDE's Qt style. Xfce has, however, been a very viable desktop in its own right for quite some time. Visually, it is very close to what people still remember from the GNOME 2 days and although its configuration utilities have seemed to lag behind the tools that other desktops offer, these days it has very much caught up. Xfce is still conceptually different from GNOME, though. It is designed to be lightweight and get out of the user's way as much as possible. Xfce does have a compositing window manager that is capable of modern transparency effects but it uses them sparingly and the environment as a whole looks and works more like the old GNOME 2 desktop than any of the other environments available on Ubuntu – with the exception of the GNOME 2 fork MATE which is not covered in this article as there is currently no easy and well supported way of installing it on Ubuntu 12.04 LTS.
Unlike the big two desktop environments and the projects reusing parts of them, Xfce's individual components are only lightly integrated and can be easily replaced with other software. It is, for example, very simple to change the Thunar file manager (which ships as part of Xfce) for GNOME's Nautilus. The user only has to go to Xfce's default application dialog, which is part of the desktop's system settings tool, and specify which file manager they would prefer to use. This makes Xfce very flexible and customisable.
One thing Xfce has in common with KDE SC, however, is that it has its own specialised Ubuntu derivative for those wanting to run the desktop environment on an Ubuntu base out of the box. This distribution is called Xubuntu and its packages are also available in the Ubuntu repositories.
Therefore, users can install Xfce by opening the Software Centre and searching for "xubuntu-desktop". This should locate the "Xubuntu desktop system" option which should then install everything that is needed to set up the lightweight environment. After logging out and rebooting, Xfce will be available as an option named "Xubuntu session" from the Ubuntu icon at the user login screen.
LXDE: Lighter still
LXDE, or the Lightweight X11 Desktop Environment, is another alternative to the established desktop environments. It has become popular in recent years and like Xfce, it is designed to be a lightweight desktop using the GTK+ toolkit. LXDE possibly uses even fewer resources, though, and is included in many distributions targeting lower spec hardware or by people who want a full desktop environment but want it to run as fast as possible. Another thing this desktop has in common with Xfce is the modularity of its components. By default, it uses the PCMan File Manager (PCManFM), but the desktop can be easily configured to use different tools throughout. Overall, the layout of the desktop it should be familiar to users who like the old GNOME 2 environment.
One of the components that makes LXDE so fast and easy on system resources is the included Openbox window manager. Openbox does not have compositing features and as such does not support real transparency of elements on the desktop. That makes LXDE fast but it also makes many effects that users are used to from desktops like Unity impossible. For users who are willing to live with this downside, LXDE provides an interesting alternative.
Similar to many of the other desktop environments mentioned, LXDE has its own Ubuntu derivative called Lubuntu. Naturally, this simplifies installation as all the LXDE packages are available in Ubuntu's normal repositories.
To install LXDE on Ubuntu 12.04 LTS, users should open the Ubuntu Software Centre and search for "LXDE". The package to install is simply called lxde. Once users install this package and its dependencies, they only have to log out and will then have LXDE available as an option in the sessions menu at the login screen.
Conclusions and things to keep in mind
For people who for one reason or another prefer something else over Canonical's Unity, it is relatively simple to install alternatives. The nature of Linux and the open source ecosystem ensures that there are plenty of desktop environments to choose from. GNOME 3's upstream desktop is available, as is the other big contender, KDE. Then there is Linux Mint's take on what a desktop shell based on GNOME should look like and, for users who like their interaction with their computer to be a little bit more old school or who value performance, Xfce and LXDE are just a few clicks away.
One thing to keep in mind, though, is that while it is easy to install all of these desktops alongside each other, there will be side effects from doing so. All of the projects mentioned in this article conform to various standards from the FreeDesktop.org guidelines. This means that most of them put their menu entries in a standard location, which enables them to include each other's utilities in their application menus. This leads to a situation where, for example, users will have several system configuration utilities and multiple file managers installed. That in itself is not bad and it doesn't break anything (or it shouldn't), but it can confuse users who are not familiar with the effects of installing multiple desktop environments alongside each other.
Users should also be aware that installing KDE SC, Xfce or LXDE using the packages from the Ubuntu repositories will change the branding of the distribution. This results in the boot screen being replaced with the Xubuntu boot screen if users install Xfce, for example.
Aside from these small downsides, Ubuntu's many derivatives – with their policy of including all of their packages in the main Ubuntu repositories – and its flexible PPA system make the distribution a good starting point for experimenting with other environments. Even if Canonical hasn't gone out of its way to publicise this since the company started to focus solely on developing the Unity environment.