If you're excited for Halo 3, you must go watch this video of Forge and Saved Films features:

http://www.bungie.net/News/content.aspx?type=topnews&link=h3savedfilms

Even if you don't care about these features, watching the footage of the new maps, weapons, explosions (etc etc etc) is a fun time.  But holy crap, if you're not excited about saved films and Forge AFTER watching this video then you are a very special individual.  Special, like "I hate video games" special.