Our preferred free desktop for X (the X Window System) is the GNOME Desktop.
There are actually a lot of reasons to prefer it over others [1]. Most of them are summarized here: GNOME in a nutshell and So, why GNOME?
Ours are mainly because we feel it is faster and looks a lot prettier than any other, and most major free softwares are already or are getting more integrated with GNOME in a near future (ex.: Galeon, GAIM, …).
It is currently available in version 2.x [2]: GNOME Desktop 2.14.
It is also particularly well integrated and up-to-date into .
Some screenshots: GNOME 2.14 and GNOME 2.8 Screenshots and GNOME 2.6 Screenshots and GNOME 2.4 Screenshots.