Australia whenever they are involved obviously, then Victoria.
AFL & VFL - Geelong
NRL - Storm & Maroons
Soccer - Melbourne Heart (When they start, for now Melbourne Victory), QPR, Juventus, Bayern Munich, Hearts (Scottish Premier League)
Rugby Union - Brumbies, London Irish, Italy in 6 Nations
Cricket - Deccan, Middlesex
Baseball - Florida Marlins
NBA - Charlotte Bobcats, like the Magic and Celtics too
NFL - Whoever Ben Graham plays for then Miami Dolphins
...and the underdog team in most cases
Don't follow many of these seriously, but I always try to have a team to barrack for in anything.