[bnoordhuis]

Xfce should speak to the HN crowd, it's the MVP of desktop environments. It's fast, uses little memory, has a no clutter interface and is pretty in an austere way.

GNOME is the kitchen sink of UNIX desktops. It has everything and does everything (the KDE crowd will disagree with me here). Slightly more polished, better integrated, accessible to the casual user.

I'd recommend GNOME over Xfce to most people but if you want a desktop that doesn't get in your face (and flies!), give Xfce a spin. http://www.xubuntu.org/ has a live CD you can try out.

[kleiba]

...and Enlightenment DR17 is the Duke Nukem of desktop environments. http://www.enlightenment.org/

[wyclif]

If you want to go even lighter and more MVP (esp. to revive an old ThinkPad or whatnot) there's always LXDE: http://lxde.org/

[namdnay]

LXDE now comes pre-packaged: http://lubuntu.net/

I cannot recommend this distribution enough.