DB-based Association Management Software?

BrynM asks: "I work for a professional association that is currently using several Symantec Act! databases to keep track of Membership, Event Registration and Continuing Education. This 'solution' is growing very long in the tooth as Act! is really just a contact database and not a full blown management system and doesn't handle payments for member dues, orders or registration fees at all. I have worked for an Association that used iMIS, which is fully featured but is way beyond my current employer's budget to implement. There's a short list of software in the Google Directory, but foraging the vendor's websites hoping for a demo and trying to get a quote for all of them would be unrealistic. Finding something that uses a database back-end that we can interface other things with would be ideal. I'm OSS agnostic on this one - we just want the most useful and practical solution. What software would you recommend? Do you have any experiences with software to avoid?"