Blogger Damien Oh discusses some of his favorite Linux window managers and lightweight desktop environments in "8 Great Alternative Desktop Managers For Linux," at makeuseof.com.
There's a lot of confusion in the Linux world about the difference between desktop environments and window managers. Oh doesn't help things by using the term "desktop managers." In short, a window manager is less resource-intensive and comes with fewer features than a complete desktop environment. Folks with older computers might prefer to use a window manager instead of something like GNOME or KDE. Folks who don't like to use the command line and who like having more well-developed applications available, and who aren't restricted by machines with low memory, and who don't want to have to figure a lot of things out, might be better served by sticking with a full desktop environment.
Some of the environments he mentions (Xfce, FVWM-Crystal, and LXDE) are light-weight desktop environments; the rest are window managers. For more info about window managers and desktop environments, see: http://xwinman.org/
In the comments following the article, some folks mentioned Openbox, which might be my favorite; others bemoaned the lack of any mention of tiled window managers like wmii. I did try wmii for a short time, but not long enough to learn to use it well. I had the feeling that tiled window managers aren't for me.
Anway, the article's worth taking a look at.