There's a plugin that allows a blog visitor to subscribe to a blog to get notifications of new posts. There's a separate plugin that allows the blog owner to subscribe someone to specific categories.
Here's a thread I started over at in the "plugins" section: http://wordpress.org/support/topic/46806#post-258831 . Since it received no replies & I thought maybe this fit into the "Feature Request" section, I'm posting a shortened version here.
I'd like to see a plugin combine those 2 features so that a visitor wishing to subscribe to a blog could choose to subscribe to the entire blog & get notifications of EVERY post; or they could subscribe to specific categories & only receive notifications when posts are written for those categories. And they could do this all in one place.