Sean,
With Ubuntu (and probably many other distros) you can install multiple desktops and choose when starting X which desktop environment you want. I have a second box that will do xfce, kde, or gnome, and remembers user preference for each user in my family. There are meta-packages in the repositories for doing this: xubuntu-desktop = xfce, kubuntu-desktop = kde, enlightenment, and some others. You can choose which to use with the "Session" button when logging into X.
Lee
Just checked, and it looks like openbox, fluxbox, icewm, and fvwm are also in the repositories. I haven't run them on ubuntu. I have tried fvwm and icewm before, fvwm with an early RedHat, but haven't used them extensively.