I know money talks in the NFP sector but I am still constantly amazed at how often the question, 'Which database should I buy?' is answered with MySQL, Zope or PostgreSQL, just because "it's free" and "you can build it yourself". So what? Aside from the fact that it's not necessarily zero cost (you still need to host it, develop it, support it, maintain it, maybe be trained in it etc), it doesn't mean it is the right answer. In fact, the right answer is, 'What do you want it to do?' But in a recent email forum where the above question was asked, only 2 people posted a response asking what the original poster wanted from the system.
Now don't get me wrong: before the open source advocates get on their high horses, I should say that I am a fan of open source software - in the same way that I am a fan of packages and SAAS. And I have seen great implementations of MySQL and CiviCRM, as I have seen bad systems. I am a fan of the right tool being used for the right application. After all, you wouldn't accept a free bicycle if you have to transport a family of four, or use a free screwdriver to bang in a nail.
And if you do want free or cheap, then open source is not the only answer. In the fundraising and membership arena, you can get packages like ZebSoft for free or KISS Contact for only £100 - and for £100 you get Gift Aid administration and direct debit support! SalesForce and eTapestry have free options for low usage CRM/fundraising; and ContactLink is a database specifically written for the voluntary sector from £400 upwards. And there are open source help desk software systems like Liberum and free packages like SomeHelp . . . (ah, okay, I admit it, I did develop that last one myself).
But before you buy or implement anything, ask yourself, what do I need it to do? And if you need help with that, then have a look at TechSoup's technology planning pages or nTen who provide lots of articles and templates you can use.
So sure, go ahead and look at Open Souce databases, but look at them once you know what you want to do with them.