The Halo 3 Heroic Map Pack is now available free of charge on Xbox Live Marketplace. To download the maps, go to the Marketplace section of the Media tab (when you press start in Halo 3's menu system) and then click and find the Heroic Map Pack and download away. The Heroic Map Pack contains three all-new Halo 3 maps:
Standoff: A symmetrical map, ideal for most player counts greater than four. A lone Spartan Laser rests at the center of the map, with each team's primary weapons being a complement of Battle Rifles, a Shotgun and a shallow clip (two rounds) Rocket Launcher.
Best Gametypes: Multi-Flag CTF, Team BRs, Team Slayer
Rat's Nest: A large, symmetrical map ideal for player counts greater than 10. There's a whole mess of vehicles at your disposal to drive around the "circuit" -- a roadway loop running around the outside of this battle-damaged human holdout -- and a variety of hallways and walkways for players who prefer their combat tunes in the key of infantry.
Best Gametypes: Multi-Flag CTF, One Bomb, Team Slayer
Foundry: It's core set-up is a symmetrical, small map, that funnels players to the center for combat and the perimeter for Sniper Rifles, Foundry's primary draw comes from its building blocks-style creation, which players can harness via Halo 3's Forge. Nearly everything you see in Foundry can be deleted, added, replaced, retuned and rebuilt into your own version of Foundry.
Best Gametypes: Whatever you design for.
For more on the Heroic Map Pack:
The Heroic Map Pack FAQ
Forging the Foundry
Quick and Dirty Guide to the Heroic Maps
The Heroic Map Pack Revealed