Grab your Gravity Hammer and Plasma Sword... it's time for DoubleXP Grifball in Halo 3. From Bungie:
"Grifball, the game created by Burnie Burns of Red vs. Blue will be an unranked, weekend-long social playlist awarding DoubleXP to winning teams. We've slightly adjusted some of the rules for Grifball (unlimited round times make for unlimited griefing potential), but kept the game's core intact. You'll be able to go in with party configurations ranging from one to eight people. This playlist will be available from Thursday, March 6 at NOON until March 10 at NOON(all times Pacific Standard). I apparently forgot to mention that the Grifball hopper is being treated like a Tournament hopper -- a hopper where there will be prizes. Tournament prizes."
If Halo 3 sports don't strike your fancy:
"Beginning March 20 at NOON will be the return of Team SWAT in the Social, DoubleXP variety. Team SWAT is an old Halo 2 gametype where players have no shields, increased body health and do not have the luxury of radar. This eight-player maximum hopper will be completely Team Slayer focused with the SWAT ruleset. This weekend playlist will be available from March 20 at NOON until March 23 at NOON(all times Pacific)."