We now have a recent post feature on the left that lets you see at a glance what is new since your last visit. Each forum will have a red dot next to the name if a new post is in the forum.
If you click the little black arrow next to the forum, it will expand and show you the last 5 posts. Each post that you have not read will have a red dot next to it.
You can expand and collapse each category, or the whole thing with the "Expand" link at the top.