Note: This has only been tested on No Mercy. Enable the "Play against NPCs only on Versus Live" glitch and kill all of the survivors. Go back to the respawn point and have all of the infected up on the rooftop. Jump down on the Hummer to have it kill all of the infected. Wait a couple seconds and you will now have a free roam camera, like on the PC version of the game. There are no secrets, but there is a mini-city of No Mercy underground that is very interesting.
To get a free roam camera without all NPC characters, Have a rigged team where four of your friends are on the NPC team and three of your friends are on Infected. Then, have all eight people jump off. They will all die and you (The Infected) should have a free roam camera. Note: This was tested on No Mercy: The Hospital. Note: This only lasts for a very short time.