Frequent forum members using this feature very often, they subscribe to interesting topics to get new replies direct in their email. Actually you need this feature if the forum is very active and without this notification you get lost between all the forum post of a particular forum. bbpress is a great (free) forum software and is based on wordpress. Since bbpress is compatible with wordpress too, blog owner like to use this forum soft with their wordpress site.
Bad news, that kind of feature is in the current version of bbpress (1.02) is not available!
There are a few plugins to handle subscribers, but the core system is based on a different idea, RSS. Most bigger forums using bbpress as forum application don't use those plugins, thats why the RSS feature needs some attention.
What does that mean for bbpress forum members?
There are different types of RSS feeds: last post (all topics and by forum), last comments/post (all topics, by forum and by a single topic), users favorites!
The first topic or forum related RSS feeds are very useful to get informed about recent posts, while the feed of users favorites needs some more attention.
bbpress has the feature to add topics to your favorites. You can find this link at the top of each topic. This list of favorite topics has it's own RSS feed showing latest posts for these "subscribed" topics. It's not the best solution but it helps to stay in touch with the topics of your interest. You can add favorites with a single mouse click and from the favorite list it's possible to remove them with also one click. A few forums might use this bbpress plugin Auto Add Favorites, if this plugin is enabled the topics the member replied to are add automatically.
So how do you know what forum software is used?
Most of the forums show the software name in the websites footer and of course the links to you forum favorites are a good indicator that bbpress is used.
List your subscribed RSS feeds in a feed reader
These days there are a lot of applications to share or list your RSS feeds in a list like Google Reader or friendfeed. Both services (and many more) will help you to get connected to the subscribed blogs and forums you like and they are a big help to get informed what's currently news or just interesting.