I don't know of one I could recommend as easy to integrate. Its something I plan to build myself at some point as part of a larger project management feature, but I have no timeline on when I will get to it.
I would look around on Codeplex for something in C#.
Yes you can subscribe to notification at the forum level instead of just threads, thats what I do so I get notification of every post. Look on the left side of the forums page for the notification subscription indicators. I've been thinking I need to do something to make it more noticable.