Some more details on Forge, the map editor for HALO 3, come from an IGN report.

• New mode/feature called the “Forge” lets you edit weapon placement, spawn points, vehicles and more in real-time during the middle of multiplayer matches. You’ll take on the shape of monitor, similar to the Guilty Spark, and will even be vulnerable to gunfire and player attacks while you edit maps in real-time.
• Multiplayer customization options let you create your very own power-ups with specific attributes. For example, if you want to create a power-up that lets you run twice as fast, kill in a single shot and jump extra high, you can.
• You can share your edited levels with others by placing it in your “shared items” slot, which other players can then access and download to their 360.
• “Bungie Recommends” section on Bungie.net will spotlight Bungie-found user content, like screens, videos, edited levels, rule sets and more.
• Heroic and Legendary difficulties now tougher than ever. Bungie recommends experienced Halo gamers play on these difficulties rather than Normal.

Hrmm. Making customizable power-ups. My only concern is NOT knowing what power-ups are in the map. I would hate to be running around Valhalla and get beaten by someone because they place their custom power-ups somewhere on the map. But, Forge is going to add at a lot of replay and fun to the game. Forge is going to drive more people to sign up on XBOX Live.