I found a article about Atlatl in the most recent copy of Make magazine - one of my favourite reads of the month.

This drawing illustrates it quite well - it is in effect a spear with a throwing mechanism that accelerates it up to speeds of 100 mph (!), and which a number of diverse cultures have used during hunting of large and small game, even for fishing.

There is a excellent website dedicated to Atlatl as a sport and technology, including links to all things Atlatl related.

A calendar of international and UK Atlatl events.