I've implemented a system which allows you to watch threads for new posts. When you watch a thread, you'll receive a notification (in the Alerts page of the menu) when someone posts in it.
You can watch (or unwatch) a thread using the new button in its toolbar, located next to the New Reply button. Additionally, you'll automatically watch threads you create.
You can view the list of all threads you're watching by using the next button in the forum index, located next to "Mark All Forums Read". Your watched threads will be displayed in alphabetical order, regardless of unread status or importance. You can unwatch threads directly from the list, too.