Debian supports all kinds of graphical environments from fully-featured desktop environments like Gnome and KDE, to lighter environments like Xfce and LXDE, to even lighter window managers like Openbox and tiling window managers like Wmii.
http://www.debian.org/devel/debian-desktop/ - Debian Desktop Homepage
Desktops and window managers
Some Window managers available under Debian:
GNOME: Simple and effective.
KDE: Complete and very customizable.
Xfce: Light and usable.
LXDE: Another light desktop.
FluxBox: Light and configurable.
Openbox: A popular window manager.
GNUStep: For those who want to discover a new experience.
Wmii: Try the « tiling » experience.
Resources
Display manager like XDM, GDM, WDM, or KDM
External links
Desktop_environment definition
http://www.enlightenment.org - Enlightenment (E16 and up)
http://www.icewm.org - Icewm