LETS and Timebanks are a few thousand communities around the world whose members exchange goods and services using a unit of account which they declare themselves.
Probably the most popular form of local currency, mutual credit systems perform many of the same functions as money, and can reduce the need for their members to use money.
These systems generally provide a directory of (goods and) services, a payment / accounting facility and sometimes other tools for local community life, such as newsletters, events, photo galleries and others.
There is almost always a committee which manages membership subscriptions, events and sometimes encourages exchange.
Other Community currency networks:
http://creditcommons.net/map Map of community currencies (incomplete)
http://communityforge.net The producer of this site
http://community-exchange.org The oldest network of exchanges in the world.
http://hourworld.org An independent provider of software for timebanks
http://http://www.letslinkuk.net The UK LETS network
http://http://route-des-sel.fr/en Accommodation network for LETS members
http://timebanking.co.uk Timebanking UK
http://timebanks.org Timebanking USA
http://timebanking.com.au Timebanking Australia