The Alterac Valley Trinket is acquired by capturing a banner in Alterac Valley as part of the Proving Grounds quest. Depending on your faction, it is called either Stormpike Insignia or Frostwolf Insignia. As a player gains reputation, their trinket can be upgraded, going from Uncommon to Rare, and finally Epic.
The trinkets give a variety of stat bonuses and equip effects, but all of them can be used every two minutes to return you to either Dun Baldar or Frostwolf Keep when in Alterac Valley.