> I'm using Ubuntu.  It booted into Gnome 2, so I used Gnome 2, which was 
> OK.  Now that they have Unity, I'm thinking about testing a few others to 
> see how I like them.  What's the easiest way to do that?
> 
> Mike

Back when I was using Gentoo as my desktop I think I used gdm as my login manager, but now most distros that I know of use slim.

Even since I can remember, at the login screen just like you mentioned in Solaris you can select which WM to login to. So, I would guess that all you need to do is
log out of Gnome and at the login screen look for what ever icon you see which will allow you to select any of the WMs you have installed in Ubuntu, regardless it be
KDE or Gnome.

I am not sure if you're being serious or not here's additional information about Slim login manager.

SLIM on Gentoo
http://en.gentoo-wiki.com/wiki/SLiM

Thread on Slim
http://ubuntuforums.org/showthread.php?t=822452