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.
Further reading: [these links need checking]
http://community-exchange.org The largest network of exchanges in the world.
http://communityforge.net The producers of this site, also offering free hosting
http://hourworld.org An independent provider of software for timebanks