Windowmaker

Also known as GNUstep. A desktop GUI for *nix-like systems. Available through ports, it is fairly light-weight (i.e., it's not very resource-intensive like KDE or gnome) and very configurable. Somewhat similar in looks to the AfterStep environment.

WindowMaker screenshot

See the webpage http://windowmaker.org/ for more information and links to other websites that have icon sets and more ways to customize the WindowMaker environment.