crunchbang

This is the most accessible way to experiment with Openbox.
See a nice review here

But a much nicer way is to follow this guide by Urukrama

Working with Openbox is addictive because of its simpleness, very productive because of keybindings and dmenu. Other good points are its ease of configuration, its wonderful looks, its fastness, low system requirements.
It's the complete opposite fo Winows Vista where everything is bloated and overdone with unnecessary complicated menu's.
On a well configured Openbox system the learning curve for pc newbies will be much and much lower than on any other configuration I know.

This is the way to go for people in education working with older hardware and possibilities to deploy a specific software setup.

Another competitor in this field is SliTaz
The wonderful thing about SliTaz is that it is the ideal on-the-road distro with a Usbstick on which you can very easily put your home partition. A problem I found that it doesn't support a very high screen resolution.
A critical review here

Just try it and you'll never regret it.