LOAF: social networking over email
Loaf is a piece of software that adds social networking to your email. Why? Because a lot of interesting things can be done (e.g. spam filtering) when you know where in your social network an email came from (or whether the sender is in your network at all). The beautiful thing is that it doesn't compromise privacy and it doesn't require you to send a huge amount of data with each email. Check it out, as they say.
Many2many also has a short post about LOAF here.