It's been a long time since I first started using Linux, and even longer since Gnome caught me in it's clutches. In that time, I've used Gnome, KDE, XFCE, LXDE, BlackBox, FluxBox, OpenBox, FVWM, JWM, IceWM and even WindowMaker. All of them had their good and bad points, but for the first time since Gnome, I've found something I like more.
E. AKA E17, or Enlightenment.
Enlightenment isn't included in Ubuntu Karmic by default, or if it is, I never noticed it before.
The useful tools of Ubuntu-Tweak and the sources generator at repogen.simplylinux.ch also don't have it listed (Although between them they cover sources for an immense amount of repositories for nearly everything).
Ubuntu Tweak's website, however, does include it. Why it doesn't show up in the app's sources centre, I don't know.
Now, Enlightenment's website does give a list of repositories for it's packages, but it's conspicuously missing Karmic. So I'll give you the instructions here to make it simple.
First you'll need the repository key, of course. Open your favourite terminal (Guake, in my case) and enter the following command:
sudo wget -O - http://packages.enlightenment.org/repo.key | sudo apt-key add -
(Note that the space on the end appears to be necessary)
Next, open /etc/apt/sources.list in your favourite text editor, and add the following line:
deb http://packages.enlightenment.org/ubuntu karmic main extras
Run a complete update, and as a good practice, I always apply any upgrades before continuing, but that's just me.
Now, to install the core of Enlightenment, just install the package 'E17'
If you're used to the flashy effects of Compiz Fusion, you can get the Enlightenment port of it by installing the package 'ecomorph-core' - note that if you do, when you choose your session at GDM, KDM or whatever you choose as your login manager, you'll need to choose the Ecomorph option, not the one for Enlightenment. Also, there aren't many Compiz plugins available, and some aren't quite the same. Try Wobbly Windows for an example.
If you want to add more functionality, have a look at the emodule pacakges. There is a dummy packages 'emodules-all' that pulls in them all, but be warned, it also pulls in the Enlightenment Network Manager, which may insist on removing your current one. If you like the network manager you already have, or aren't sure, pick and choose your emodules by hand.
There's a few other packages of note with it. Empower is like gksu, though I've never actually got it to work. emprint, and it's corresponding emodule is the screenshot tool. There's more to, of course.
It's fairly easy to customize too, with an entire section of the OpenDesktop site to itself, namely, e17-stuff.org - though there isn't much there in comparison to Gnome-Look and the like, it's enough to get started.
With that, I've found that Enlightenment without EcoMorph is a very good desktop for older computers, while including it provides a nice balance between usability and special effects.
Overall? E17 is an impressive desktop environment, and if you're not certain what Window Manager or Desktop Environment to use, give it a shot, and give it a fair trial. It takes a little getting used to, and it also takes some customising to get it how you'll like it, but it's definitely worth a good trial.