"CiviCRM":http://civicrm.org/, which is, as they say, "a powerful contact, fundraising and eCRM system that allows you to record and manage information about your various constituents including volunteers, activists, donors, employees, clients, vendors, etc. Track and execute donations, transactions, conversations, events or any type of correspondence with each constituent and store it all in one, easily accessible and manageable source."

I've been watching it develop, and they've now started to "document their architecture":http://civicrm.org/node/91

It's worth a look.