Nothing incites the killing urge like the taste of one's own blood.[1]
Berserker Rage is a free self-buff that renders the warrior immune to [Fear] and Incapacitate effects, notably [Sap], and causes extra Rage to be generated when taking damage. It lasts 10 seconds, and has a 30 second cooldown timer.
[Intensify Rage]: reduces cooldown by 11/22/33%. (20.1 sec cooldown at 3/3)
Tips and tactics
Berserker Rage is usable in all stances, and should thus be used as needed: to break fear, sap or incapacitate, or to generate extra rage from damage taken when under attack (tanking, PvP, solo).
It is worth noting that it does not actually create rage itself (unless using [Improved Berserker Rage]), so to get the most out of it, you will need to have aggro.
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 changes
Patch 3.2.0 (2009-08-04): Tooltip clarified to indicate this ability breaks existing Fear, Sap, and Incapacitate effects.
Patch 3.0.2 (2008-10-14): Berserker Rage is now usable in all stances.
[Improved Berserker Rage]
[Fear] and Incapacitate effects, notably 
[Sap], and causes extra Rage to be generated when taking damage. It lasts 10 seconds, and has a 30 second cooldown timer.
40
[Intensify Rage]: reduces cooldown by 11/22/33%. (20.1 sec cooldown at 3/3)
