org.civicoop.civirules
Native CiviCRM extension CiviRules
A cookbook on what this extension could be used for: http://wiki.civicrm.org/confluence/display/CRMDOC/CiviRules+Cookbook
More technical details and tutorials how to create your own stuff: http://wiki.civicrm.org/confluence/display/CRMDOC/CiviRules+-+how+to+make+your+own