A bit about Fluxbox

Fluxbox is a highly-configurable window manager for the X Window System that was based on the Blackbox 0.61.1 code. It has full of features to make an easy, and extremely fast, desktop experience.

Fluxbox requires less time for booting and uses considerably fewer system resources than, for example, KDE or GNOME. This suited for situations where system resources, especially memory, are very limited. I use Fluxbox on my ASUS EeePC 701: 900 MHz CPU, 512 Mb RAM and yes – only 4 Gb HDD :/