CiviCRM is one of the more interesting and intricate of the free CRM's available. If you are a non-profit, this CRM is for you! Built specifically for this sector, CiviCRM will handle all event transactions, membership signups (including renewals, different membership levels and more), registration, donation, reporting, accounting, emails (no need for a Constant Contact - Civi retains all contacts - thus your email list is always automatically updated!), other contribution forms, paypal or authorize.net integration and more.
CiviCon, the yearly convention for programmers, users and administrators of Civi, allows for all disciplines involved to get together to improve Civi's functions, capabilities and security. Civi is 100% open source - any updates are funded by larger non-profits or through crowd-sourcing on the civicrm.org website. You can pick a project you wish to fund and send money the developers way.
I currently have two clients using this software. While there is quite the learning curve, it's potential is phenomenal! Anyone in a fundraising position can use this software to track contributions (sources, amounts, frequency) and set up automatic renewal emails and more to maintain contact with the non-profit's base.
Visit CiviCRM.org for more information.