One of my great disappointments with Google Sites is the fact that it doesn't have a threaded discussion feature. I have been told that this feature will be added in the future, but for now, it's only a dream. For classroom teachers this is a problem because discussion forums are a great way to encouraged discussion and debate outside of class.
My initial disappointment gave way to excitement when I discovered the Tal.ki Sites Gadget. This third-party Gadget allows you to embed a threaded discussion into your Google Site.
The downside to Tal.ki is that it does require authentication. The good news is that you can use one of numerous login credentials (Google, Yahoo, Facebook, etc) to do so. Chances are good that you and your students already have at least one of these. Google Apps accounts will work as well.
Here's how to embed a Tal.ki forum into your Google Site.