Alternative WM's/complete desktop environments:
icewm
LXDE - Lightweight X11 Desktop Environment
Openbox
.:F L U X B O X:.
Equinox Desktop Environment (EDE)
ROX Desktop

Addons for light desktop environments:
fbpanel
LXPanel

eerielinux - Linux desktop comparison summary – 20 solutions for your desktop!

eerielinux - Linux desktop comparison (pt. 1): Modern GTK+ DEs
eerielinux - Linux desktop comparison (pt. 2): Traditional GTK+ DEs
eerielinux - Linux desktop comparison (pt. 3): Qt-based DEs
eerielinux - Linux desktop comparison (pt. 4): Less common GTK+ DEs
eerielinux - Linux desktop comparison (pt. 5): Exotic DEs