As I said two posts back we use the Community Edition of SugarCRM as our Customer Relationship Management system. SugarCRM is very good and it has a good community behind it. There have been a lot of modules written that extend SugarCRM in various ways, many of which interact with other online systems.
One of the more useful Open Source tools we use is SugarCRM. SugarCRM is a Customer Relationship Management system (CRM). A CRM is used to track and organise contacts with current and potential customers or clients. We find SugarCRM invaluable for this and use it to store information about every contact we have with our clients.