Re: This could have been better
There's a ton of WM's out there but there's only 4 real options for desktop environments; KDE, GNOME, CDE & XFce.
There's also GNUstep (http://www.gnustep.org/). It's an object oriented framework for developing user applications, and there are already quite a few apps. It also aims to be a complete desktop solution one day. It's source-code compatible with Apple's Cocoa frameworks used on Mac OS X.
As I mentioned in a post above, there's an Objective-C WM being written for GNUstep called InterfaceWM (http://interfacewm.sourceforge.net/) that is just getting started.
Re: Same old same
One WM I haven't seen mentioned much (mostly because the project is still in its infancy) is InterfaceWM (http://interfacewm.sourceforge.net/). It's a window manager written entirely in Objective-C and taking advantage of the excellent GNUstep frameworks.
The project was stalled for a while but two new developers have come on board recently, so things should start moving along pretty soon. Not ready for prime time yet but keep it in mind if you're interested in seeing a Linux-kernel-based NeXT clone. And when the GNUstep theme engine matures, maybe a Mac OS X clone too...