This weekend is a double XP weekend for Halo 3.  If you hop online and play the Social game type grifball you'll get double XP for all the matches you win.  Grifball is a community created game type that is loosely based on soccer and played with the gravity hammer.  I played some Halo 3 last weekend playing the Team Swat for double XP and ranked up.  Since you can party up with friends it's a good way to play with people you like and avoid all the idiots online.

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xbox360fanboy - halo-3-double-exp-weekend-grifball