This add-on plugin for bbPress will allow anonymous users to subscribe to topics and get email notifications when a new reply is posted. The notification email includes an unsubscribe link.
bbPress notifications will keep to go out to registered users, this plugin only extends the thing to anonymous posters as well!
A case example with >100% subscription rate
This is vital for support forums, for example. On Post Pay Counter support forums, I did not want customers to sign-up: I wanted them to be able to request support in a matter of minutes, without any hassle. I liked the idea of “enter your name and email and you’re done!” But I also felt like they needed to be notified when someone replied to help. It was not compulsory, of course, but I would have wanted it as a customer.