Berserker Rage is a core warrior ability learned at level 54. It is a free self-buff that renders the warrior immune to Fear and Incapacitate effects, notably Sap, for six seconds.
This skill not only grants immunity to fear, sap and incapacitate effects, but also breaks the warrior out of it.
The immunities can help immensely in PvP combat, where any class who can uses Fear often. Carefully timed Fear immunity can really ruin a Warlock's day, and the Knockout immunities help defend against several Rogue abilities as well as Repentance.
Patch 3.2.0 (04-Aug-2009): Tooltip clarified to indicate this ability breaks existing Fear, Sap, and Incapacitate effects.
/Patch 3.0.2 (14-Oct-2008): Berserker Rage is now usable in all stances.