Light GUI user interfaces

Openbox and Window Maker are great suggestions. You may also have luck with related projects and derivatives; I had a good experience with Fluxbox on Crunchbang (Ubuntu-based). Lubuntu, with the LXDE desktop environment is very lightweight while providing a bit more out-of-the-box infrastructure than going without any desktop environment.

Though, I've recently dumped all of the above for XFCE, which satiates my desires for a GNOME2-like setup. There is a good community as well, and there's relative interoperability with GNOME2. For example, GNOME2 panel applets work in my XFCE panels. In general, XFCE gets out of your way and lets you focus on your work.

Also, for what it's worth, Linus Torvalds recently switched to/advocated XFCE in response to the GNOME3/Unity transitions taking hold in many distributions. (Slashdot 2011)

If you want ultra-lightweight, try a tiling window manager. You might start with Awesome, Enlightenment, or StumpWM. (bigger list at Wikipedia)