The Wikipedia page on Comparison of X Window Managers sorts the various Window Managers into four categories: Heavyweight, Middleweight, Lightweight, and minimal. You'd probably be interested in those in the minimal category.
Right now, those include Matchbox, sithWM, evilwm, dwm, WMFS, wmii, and scrotwm. (i3 gets put into Lightweight; Xfwm (used by default in Xfce) and Openbox (used by default in LXDE) are both considered Middleweight by this classification.)
I don't know what grounds were used to sort these out, and haven't tried enough of these to know how accurate it is.
Regarding the point you made about Compiz standalone, however, I would not consider that more lightweight than using LXDE with Openbox or XFCE with XFWM. Compiz is a resource beast; using it alone is not going to be much different than using it under GNOME, so long as you don't load anything in GNOME you don't need.
(Remember that XFCE and LXDE are desktop environments; you could run their default window managers without using the rest of these desktop environments, just like you can run Compiz without a Desktop environment too. Well, at least I know you can run Openbox without LXDE. I've never tried running XFWM without XFCE.)