On of the great new features in Halo 3 is the addition of the Halo 3 Theater.  This part of the game allows you to watch multiplayer matches you've played in and gaming sessions you've had in the campaign.  Each game session (mulitplayer and campaign) is temporarily saved on your XBox 360 and from these you can either save the entire session, record a clip or take a hi-res screen shot.  These can then be uploaded so that friends with Halo 3 can view your movies, clips and/or photos.

I have seen other games that have allowed you to take pictures and view online matches but never in a first person shooter.  While viewing a multiplayer match you can actually switch who you are watching.  So now you can review your online matches and see exactly how player X got the drop on you or where he was sniping from.  By watching these matches you could actually study what the good players online are doing and try to get better by copying their tactics. 

Now this feature may not be revolutionary to gaming but it is to the FPS genera.  Bungie has raised the bar on now a multiplayer game should be judged again.  I did find the UI a bit hard to figure out how to save and t make sure everything was saved.  Still this is a cool little feature to have.  I've shared a couple matches and some screen shots.  Right now you have 6 slots and/or 25MB limit.  For a fee you can get more if your really want to share your stuff.