CouchSurfing
A site where travellers can find 'free couches' to sleep, offered by volunteer participants worldwide.
CouchSurfing is a worldwide network for making connections between travelers and the local communities they visit.
URL = http://www.couchsurfing.com/
More Information
Read their about/faq pages for detailed info on how it works, at http://www.couchsurfing.com/help.html
Part of the Hospitality Exchange Networks and a P2P Exchange Infrastructure Projects
See also their Center of Adventure Economics