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.
Now, after a year from bbPress - Anonymous Subscriptions release, we have seen a 139% subscription rate! On a total of 117 topics posted since the plugin started working, we have had 163 subscriptions to topics. However, it often happened that more than one user subscribed to a topic. On average, we had 1.7 users subscribed per topic.
How is it possible that we registered a >100% subscription rate? Because users also subscribed to older topics!
I would never go back without having this plugin on a support forum, or anyway on a public forum in which most posts are from anonymous users.