There are several tools, none of which do exactly what I think Brian Hanley is asking, but all have some utility.
  • New Posts - top blue bar menu, in the middle (when signed in). Lists posts that you have not read (and are less than 10 days old). Across all forums that you have privileges to see. This sounds closest to what I think you are asking. We just enhanced this feature a few days ago, so it now provides better tracking of what you have read (post-based, not session-based).
  • Recent Posts - top left, under the donkey. Lists posts within the last 24 hours.
  • Subscribe to a forum or thread. Select drop down menu Forum Tools or Thread Tools, towards the right side of the blue bar at the top of the forum or thread display. In conjunction with some settings there and your UserCP, this provides for e-mail notifiation when there are posts in your subscribed forums. You can view and manage subscriptions from your UserCP.
  • External feed to a feed reader. See this thread. Options to sort and view are based on your feed reader. Note that this method only provides feeds for those forums that our unregistered members can see, so nothing in the Members Only area goes out. Provides the last 15 posts to your feed reader.
Keep an eye out for the icons on the left of most forum display pages. There's a legend at bottom. They show where the new posts and unread threads are.

I need to make the RSS, Atom etc. links visible and available. There's a little bit of info on the rest in the FAQs.