The Lightweight X11 Desktop Environment, or LXDE as it's more commonly known, really takes its "lightweightness" seriously. They claim that LXDE's minimum hardware specifications are only slightly higher than that of Windows 98, consuming only 45MB of memory after boot. Any Pentium II processor should be capable enough of running LXDE, and thanks to it being ported to ARM and MIPS as well, it can serve a whole range of machines, like the Chinese MIPS netbooks.
The LXDE interface is attractive but simple, more or less similar to Windows' Classic interface. It comes with a whole set of basic utilities and configuration tools, so LXDE really stands on its own two feet. It's installable on most modern Linux distributions, with some of them even releasing specific versions with LXDE or some of its components.
Due to the success of LXDE, there's even talk of an official Ubuntu derivative, Lubuntu. In an interview, LXDE president Mario Behling detailed how he met Mark Shuttleworth to talk about this possibility. "Mark is pretty nice and he understood immediately, that LXDE as a desktop environment could be the base for an official derivate. This was only an initial talk and we need to understand that there is still a long way to go. The conversation was mainly about how we could work together as a community," Behling explains. As of yet, however, there is no financial commitment from Canonical.
It seems like LXDE has the possibility of becoming the desktop environment of choice for resource-constrained machines. I have several Pentium II machines that would perform pretty good with LXDE as their desktops, and I must say, t has become a tempting idea.