I have been recently been looking into desktop managers other than Gnome and KDE. Of course, there are Ubuntu distributions with different desktop managers, including the "standard" Gnome, Kubuntu with KDE, and Xubuntu with Xfce. But I don't want to reinstall just to try a different desktop, and heaven knows I have enough bootable partitions on my laptops without adding several more just for this...
Fortunately, with Ubuntu (and probably others) it is relatively easy to download and install several such desktop managers, and then choose between them on login. There are meta-packages for both xfce and lxde, so you can simply go to Synaptic and mark the metapackage, accept the (rather long) list of additional packages to be installed, and then Apply the changes. In a matter of a minute or two you will have them installed (and it appears that lxde brings along most of openbox as well).
Once they are installed, you can log out (or reboot) to get back to the login screen. Then click "Options" at the bottom left corner of the login screen, and choose "Select Session...". That will present a list of sessions, which will include Gnome, Xfce, Lxde, Openbox and a few others. After you select one and login, you will have the opportunity to specify if this is a one-time selection or should be made the default session for future logins.
The idea behind all three of these alternative desktop managers is that they are supposed to be smaller, lighter and faster than the standard Gnome or KDE desktops. My experience with them so far varies, but I suppose they generally follow the old saying "you have to give up something to get something". I have been testing them on my HP Mini-Note netbook, on the idea that if the most important advantage should be better performance and less system load, I should see it most easily on a supposedly "under-powered" netbook.
Xfce seems to be the most advanced of the three, which means you aren't giving up much and (surprise) you don't gain much in terms of performance. I like Xfce, I think the look is a bit "cleaner" than Gnome (try to quantify that one!), and although it doesn't have all of the applets and controls that I normally use, it looks like it has all of the most important ones. However, I don't see any significant difference in speed between Xfce and Gnome, either on login time or in menu/icon selections, program startup or whatever. If I were to decide to use Xfce in the future, it would be because I like the "look and feel", not because of performance considerations.
LXDE seems to have stayed much closer to the "minimalist / performance" philosophy. Both of those things jumped out at me the first time I tried it - login was much faster than with Gnome (or Xfce), and the default desktop was extremely sparse. As with Xfce, it looks like everything you really "need" is there, but there are even less options, applets and accessories than Xfce, and fewer configuration and customization possibilities. But it still looks decent, and everything about it just feels a bit faster than either Gnome or Xfce. My summary on this one is just the opposite of Xfce - if I were to choose to use this one, it would certainly be due to the speed advantage, not because I just "liked" it.
openbox seemed to get installed along with LXDE, I assume because the latter is built on it somehow, or uses some part of its functionality. In fact, that pretty much sums up my impression of openbox - it is so minimalist that it seems to me that it is better suited to be used as a base to build something else on, rather than being used as a desktop management system itself. In fact, the first time I tried it I didn't even realize when I had gotten logged in, because all it gave me was a blank screen - and it did that REALLY fast. But when I right-click the mouse, I get a minimal menu system, with choices to start a web browser, terminal emulator, or the openbox configuration program. I assume that with some time and effort, I could learn to configure openbox much more to my liking, but it is really off-scale below the requirements that I am looking for right now.
For the time being I am going to be using Xfce on the Mini-Note, just to gain a bit more experience with it. As I figure out a few things, I'll probably switch to Lxde to see how it compares, whether it has comparable configurability, applets and the like. I have a feeling I'll still end up staying with Gnome, though, because even on the Mini-Note the speed difference doesn't seem to be enough to justify having to deal with different desktops on different systems.
There is a lot more to desktops than what I have considered so far here - primarily the look and feel, menus, applets and configurability. Gnome and KDE come with a whole range of utilities and applications, from CD/DVD burners to Office suites, to graphic programs, file managers, web browsers, mail and calendar managers, and much more. These lightweight desktop systems generally don't include nearly as much, but they can often use programs and utilities from either Gnome or KDE to supplement what they have.