This ability is usually the best way to hold a mob's attention due to its very high Threat-to-Rage ratio. [Sunder Armor]/[Devastate] are available on command and can be spammed (given sufficient Rage, of course), and [Shield Slam] generates more threat per application, but smart tanking warriors use Revenge at almost every opportunity.
[Shield Block] is a popular way to enable Revenge by guaranteeing a block within a few seconds.
Revenge may seem similar to the Battle Stance ability [Overpower] (being a situationally-reactive skill), but there are important differences. First, Revenge is available after your Warrior blocks, dodges, or parries; Overpower only after your target dodges. Second, Revenge has a flat amount of damage modified by AP, while Overpower is straight weapon damage. Third, [Improved Revenge] gives you a chance to hit another nearby target, whereas [Improved Overpower] increases critical-hit chance.
Note that you do not need to Revenge the mob that you blocked/parried/dodged: once Revenge is lit up you can use it on any target until the timer expires. This can be a very Rage-efficient way of building threat on multiple mobs over a short period of time: if multiple mobs are targeting you, Revenge should activate regularly, particularly if your defensive skills are high.
Note that with 2/2 [Unrelenting Assault], Revenge will be on a one-second cooldown - while a parry, dodge, or block enables the ability for a few seconds - effectively allowing multiple Revenges to be used after a single dodge, parry, or block. It is unknown whether this is a bug or working as intended.
Patch 3.3.3 (2010-03-23): Damage done by this ability (base and scaling) increased by 50%.
Also, Improved Revenge can no longer trigger a stun, taking Revenge off the list of warrior interrupt abilities.