"XFCE, short for XForms Common Environment, is considered to be
the most popular lightweight desktop environment available on
Linux. It uses a surprisingly low amount of memory (especially when
you come from a Windows perspective), while at the same time
looking good and working just as well as any other Linux flavor. As
such, it is a popular choice for machines that don't have a lot of
hardware on them, so you'll still be able to use a machine for your
enjoyment that has, say, 256MB of RAM or less.