Eavesdrop

Eavesdrop is a Call of Duty 4: Modern Warfare 3rd tier perk that gives a player the ability to listen to enemy voice chat. Eavesdrop will only work when a player is in close proximity to enemy players. A player will be able to hear both allied and enemy voice chat simultaneously.

Tactics
Eavesdrop is perhaps the only perk which is renowned worldwide as useless because enemies can simply mute a player, preventing them from being heard by that person.

It is also ineffective because players will often be in parties, not have headsets or at least not talk about anything which would benefit the user, such as where other players are going, tactics etc.

There is, however, some comedic value to be had with "Eavesdrop." If you come across a group of particularly loud players (especially younger ones) then using the knife melee on them can yield some hilarious results as they scream in indignation. Clearly this is not a recognized military strategy.

On the PC, it's nearly useless, as players rarely use the in-game microphone. Or as many uses different programs on the side to communicate, which the Eavesdrop ability of course doesn't pick up.

The PS3 version is also useless, since there aren't a whole lot of people with Bluetooth microphones.

By general opinion, it is the most useful on the Xbox 360 version, since microphones are more readily available than on other gaming platforms.

Eavesdrop seems to be more useful in hardcore, because when throwing a grenade or fireing into a crowded area, a player with a microphone is likely to inform his teammates before preforming the action, and thus notifying any nearby player with eavesdrop

Eavesdrop isn't as popular as in the first days of the game's release on the Xbox 360, as there was an update including Xbox LIVE parties, therefore a lot of people use them when they play as a party with friends in order to avoid non-team-members hearing their chat.