Complementary Currencies for Drupal

Context and Purpose
Mutual Credit system as part of a social networking site. This module provides a transaction form, and account display data, as well as a transaction API and a log of all transactions. Also includes features like stats, mass transactions, and offers/wants.
The module has great plans, particularly to improve the API, implement intertrading, SMS integration, a companion module to enable users to guarantee each other's credit limits.
See the modules own page for more info: http://drupal.org/project/mutual_credit