I've been recently looking for a service that could be used as a medium of communication within a community. I do not want to type that I was looking for a community server as it is name of a product but I was. I needed it for Cracow .NET User Group that I am member of. I considered:
Each of them has different set of features and can be applied to different requirements. All of them fullfil my requirements. I finally chose google groups because of lack of any additional cost but the Community Server is very interesting. I installed it (xcopied) at home and try to use it with my wife.
If you need a server for community look at the Community Server.