PLAY ON XBOX LIVE. It the funnest freaking thing ever. Look for Melvin Spar989 if you do.
The coolest one would have to be the flying one. You need two people, in a level with warthogs and banshees, and you need to have swords. One of you get in the passenger side of the warthog and the other in the banshee. The banshee needs to fly in close to the guy in the warthog, that guy needs to slash at the guy in the banshee when the crosshairs go red (but stay in the warthog), you won't see it but you'll hear it. Now the banshee needs to fly as far away as possible and you stay in the warthog. When he is a ways off you press X to get out of the warthog and you should fly towards him.
Another would be on Zanzabar, you get caught under some glass in the right window in the front above the gate, and then you jump off and when you hit the ground you will bounce really high. This also works in Coagulation, but it is really hard to explain for both cases, some one on xbox live needs to show you and walk you through it.