My favourite halo mod that I make is so damned entertaining XD What you do is pop open Halo in sparkedit, and then place a warthog into the white area outside the map, but just outside, not far, but at that highest cliff point. Then make a teleporter somewhere so you can get up and start halo. Access your teleporter, hop in your warthog, drive back a bit so you have some run and then go flying off the side of the map
It's the funnest thing ever. It isn't possible on the PC version too because invisible walls keep you from doing things like that.