I've been trying a lot of window managers with Ubuntu in VirtualBox over the last few months. These are a bit harder to get set up than the regular desktops such as Gnome or KDE, but they're a lot more lightweight. If you have an older computer that might suffer under the weight of Gnome or KDE, then using a window manager instead is worth considering.
I'm a fan of Enlightenment, but I don't like the current stable version (E16), and the forthcoming version, E17, has been in development for years, despite its use in OpenGEU and gOS (although gOS Space has switched to Gnome). Enlightenment is probably one of the biggest and most powerful window managers, yet looks amazing. I've heard rumours that there may be a release of a stable version of E17 this year, but I'm not optimistic. However, it really does look great - I actually think it knocks the stuffing out of the Mac desktop.
So, Enlightenment's not really much of an option for me. I've tried Fluxbox, Openbox and several others, but one stood out for me - IceWM. This is a very simple window manager - it resembles the classic Windows GUI, with a main menu at the bottom left and a panel along the bottom. Although there are a few graphical configuration apps for it, it's not too hard to configure it by editing text files. It has a great integrated application launcher - just press the Windows key and the Space bar, then type the name of the application and press Enter. It doesn't autocomplete like Google Desktop or Katapult, but it's still a great touch.
It supports both Gnome and KDE apps, and it's easy to find or create themes for it - you can find plenty of Windows ones, as well as more than a handful of Apple-themed ones. Presumably because of its resemblance to Windows, IceWM is used in the basic desktop on an Asus Eee PC (using the SilverXP theme), making it an easy transition for people used to Windows.
I've found IceWM to be a great desktop, and have now installed it alongside KDE. If your computer finds Gnome, KDE or even Xfce to be a bit too heavy, IceWM is definitely worth a try.