Stop Abuse For Everyone (http://www.safe4all.org) uses OpenACS for a number of reasons. First of all, it is pretty easy to use once you get it set up. Second, it's very powerful, and does most of the things we want to do without much customization. Third, it is free, but being actively worked on by a very responsive, helpful community of software developers.

For a non-profit, it's good because it doesn't require a lot of maintenance once it's set up, and doesn't cost anything.