Bungie has been releasing maps for Halo 3, most recently, the Legendary Map Pack. The recent Map Pack was the Heroic Map Pack which came out sometime last year. It became free only recently and you can downlaod it from the Xbox Live Marketplace. Either access it directly from your Xbox Dashboard Menu or while in Halo 3 (must be connected to Live). I suggest you access it from the game directly because it autmoatically brings you to the Halo 3 download page. Scroll down the menu until you see the Heroic Map Pack, it should say 'Free' next to it. Press it and continue. The download should take roughly 30 minutes on a good connection. You will now find you're able to play matchmaking and 3 new maps have been added to the originals, Foundry, Standoff, and Rat's Nest.
