As a ref I have seen a bunch
"Jehad Scream" - you know the one that goes "lilililillililililili" and involves shooting a bunch of rounds into the air
"Rebel Yell" - Yell as you charge down the field and hope the noise you make drives the other team back some
"For _____________" - This one is a good open ended yell. I few that I have heard are:
The Overmind
The Hoard
Our Bonus (Corporate outing)
World Peace