OpenBox is well known for it's minimalistic appearance. It is just like any other *box WM, but provides a lot more options then the other *box WM's do to change the appearance and themes.

As a lot of the applications for Ubuntu are designed for either gnome or KDE users, Openbox WM provides a suitable environment to allow the applications run how they should do. As well as this, you can run the Openbox inside the Gnome or KDE environment. - What are the advantages of this? - Not only can you have your familiar Gnome/KDE appearance, you also have the ease of using OpenBox alongside. Being minimalistic, running OpenBox within Gnome/KDE will reduce the amount of RAM that it takes to run. What could be better?

To install OpenBox, extra themes, menu editor and the OpenBox configuration GUI:

sudo apt-get install openbox obmenu onconf openbox-themes

All images from: http://www.lynucs.org/?openbox

Below are a few of the applications that I run with OpenBox:
Conky - System monitor
Feh - Image viewer & background manager
gDesklets - Display widgets on your desktop
PyPanel - A very customizable, light-weight panel.