File Share

The File Share is a feature in Halo 3 which allows players to upload custom content for others to view and download. This content include game films, film clips, map variants, gametypes and screenshots. By default, players may upload up to 6 custom content items, but may subscribe to "Bungie Pro," which allows them 24 upload slots for 750 Microsoft points. For a limited time, Bungie Pro was free for 4 months with the purchase of the Legendary Map Pack.

Also, by becoming a member on bungie.net you get 18 free folders for a limited time to place whatever you want.