CiviCRM

This excellent open-source solution is free, powerful, supported by a broad community, and able to do a wide variety of critical CRM tasks that all small organizations need. Membership and mail list are completely integrated. These were more important to us than a classic "marketing/sales" CRM features. Civicrm is the MOST flexible product I have ever worked with - it will do WHATEVER I need!

The flexibility of the system can lead to a certain complexity and concomitant learning curve. I dislike the scarcity of user-friendly, and up to date documentation, which causes a greater reliance on the services of consultants than should be necessary. The fonts and some of the formatting is not user friendly. Once I've gotten the basics down tasks can be completed in short order!

Dive right in with their online demo. If you're not very comfortable with administering sophisticated web applications, then consider going with a hosted solution. Implementing CiviCRM as a membership system requires codifying rules -- and in many small organizations, rules are ad-hoc. This may result in some unpleasant discussions, but ultimately, it's worth the effort.