Besides having one of the most feature rich games ever made Halo 3 is backed by Bungie.net with some of the most detailed stats and articles about the game you can find.  Each Halo player that has a Live account has their game stats sent to Bungies web site.  The site has a more detailed area you people to see your service records, shared pictures, videos and general profile choices.

The part I love is their service records which keep track of all of your campaign stats and multiplayer matches.  Now you can go online and see how many times got killed in a campaign, how many kills you got and how many times you betrayed your allies (the dude just walked right into that sticky grenade! I swear!).  There is also a more detailed career page that will show you your meddles earned online and what your experience ranking is and what is your next promotion.

When viewing a person's online profile you can also see some of their shared content.  Bungie allows you to upload six items for free so others can view them.  To watch a clip or movie you must download them to your 360 first.  You can view the high res screen shots. 
Below is a link yo my Halo 3 profile so feel free to check it out and see all the cool stuff Bungie has thrown onto their web site.  There are great articles about campaign scoring, skulls and other aspects of playing Halo 3.  You can also see a picture of my online profile wearing the special Hayabusa armor I have unlocked.

http://www.bungie.net/Stats/Halo3/Default.aspx?player=voghan