One of the questions I get asked pretty frequently by non-Linux users is "why are there so many desktops?" Mac and Windows users don't have to choose between desktop environments, why should Linux users?
GNOME and KDE get all the attention these days, but Linux users looking for a lightweight desktop environment would do well to consider Xfce. The project has come a long way since the days it was a clone of the hideous Common Desktop Environment (CDE), and is still going strong, and came out with the 4.6 release today with a ton of improvements. Xfce is a prime example of why duplication is sometimes a good thing in the FOSS arena.
Common wisdom, usually outside the Linux community, holds that the Linux desktop would be in much better shape if everybody worked on a single desktop rather than several alternative projects. Of course, this is sort of like assuming nine women could produce a baby in one month. Even if one of the desktop projects disbanded, there's no reason to assume that the developers would all fall into another desktop project and become as productive as they'd been in the previous project.
Each desktop has its own personality, goals, strengths, and weaknesses. Xfce doesn't try to be as full-featured as GNOME or KDE, but instead focuses on being a lightweight desktop -- and does so very well. GNOME and KDE may be more mainstream (in as much as any Linux desktop is mainstream at this point...) but having choice is more important than uniformity to the larger community.
This is where commercial interests and community interests diverge a bit, since most ISVs and vendors want to have a single target, and certainly not three or more. However, it's better for the community as a whole to have the options, and the Xfce folks have done a good job of ensuring that apps written for KDE (those utilizing Qt) and those written for GNOME (GTK) run just fine on Xfce.
It's good to see Xfce still chugging along. I can't wait to fire it up on my openSUSE 11.1 system and give it a whirl.