I often use:
Gnome, Fluxbox and Xfce. Gnome with Compiz/XGL
Flux is lightweight and has everything you need and nothing you dont.
Xfce is lightweight and adds a few extra features, but personally I am not a fan. I use it on my laptop when other people want to use it.
Gnome is the one I use on my main computer since I dont need anything lightweight. It just looks better!