Enlightenment is very customizable, however the current stable release 0.16.5 i think is buggy and slow compared to fluxbox, but very cool nonetheless, and still faster than KDE/GNOME, but if you have the bandwith and tiome download the development CVS version that is really awesome, and when it is released as stable, im convinced it will enjoy a lot of popularity.
If you dont want to mess with development stuff, then fluxbox. It's small, lightweight, very fast, and customizable to a certain point. You can change themes and hack some things here and there in the ~/.fluxbox/init, but you can't change window frames, save sessions. have different wallpapers for each desktop, have virtual screens AND virtual desktops together, etc.. like in Enlightenment.
More info about Enlightenmnt 0.17 (AKA development version) can be found on their site www.enlightenmnt.org