Call of Duty 4 Perks

Tier 1 Perks

C4 x 2
C4's are a tricky item to use, but if done correctly, can be absolutely devastating. The trick to using C4's is to reach a highly active contact point early in the map. (Example: Grassy lot near the apartment in Strike [Fact Check, anyone?]) By tossing these near enemy entrances, they can be detonated at will, unlike claymores. [Need more information, I've never personally used these before, just going off what I've seen.]
Special Grenades x 3             
Triple special grenades gives you three of either stun or flash grenades, depending on what you picked under your 'special grenades' category. You CANNOT receive three smoke grenades, as that would just get annoying. Nothing much to say here, but compared to other perks in this tier, probably not the best option.
RPG-7 x 2 This perk is more useful in Hard Core mode than in normal mode because the RPG will not do enough damage (besides a direct hit) to kill a person in normal. However, in Hard Core, an indirect hit (say, hitting a wall someone is standing in front of) will kill a person. Also, this perk is the best way to take out helicopters with 3 rockets taking out a normal helicopter and 2 rockets taking out a Hard Core chopper.
Claymore x 3
This perk is probably the most used in all of Tier 1, due to the fact that you can plant it and forget it, and usually receive kills for it. The trick with claymores is to plant it close enough to a door so that the blast will kill and entrant, but not close enough that it will be noticed before entering. The best place to plant a claymore is usually about 2 feet in against the wall near a door, angled slightly into the room so the blast will still kill a running entrant. If the claymore is pointed straight at the door the directed blast may not kill a sprinting player. This perk is best used by snipers, as it can protect the entrance to your sniping spot, preventing a embassing knife while scoping.
Frag x 3 Two words: NADE SPAM. Usually frowned upon in multiplayer because of the lack of skill involved, but an accurate nade storm can kill a fair amount of enemies. Just be prepared to be flamed.
Bandolier Choosing this perk will give the player two [fact check, not sure about this] extra magazines of ammunition. Useful for camping out behind enemy lines, where moving to recover downed players ammo is not an option.
Bomb Squad The bomb squad perk allows you to see enemy placed C4's and claymores, a very useful perk for the 'bunker clearing' player who likes to take out enemy snipers in order to advance team mates. Because snipers often choose claymores in order to defend their positions, choosing this perk will allow you to destroy their defenses and take them out.
Tier 2 Perks

Stopping Power Stopping Power is one of the more useful perks in Tier 2, as it increases your bullet damage by approximately [x]%, leading to faster kills. Because of the increased damage already present in Hard Core modes, it is not recommended to use this while playing HC.
Juggernaut Juggernaut doubles the damage a player can take before dieing. Using in Free-for-all due to the fact that you may be attacked from multiple hostiles at once.
Sleight of Hand This perk doubles the speed at which you reload, which is the most useful with classes that have LMG's, as they take forever to reload the normal way.
Double Tap
Double Tap increases your firing rate on all guns. I find it not very useful because almost always, 4-5 shots to the chest will kill a player, which you can easily get off in under a second.
Overkill This perk allows you to start off each round with two rifles, and no pistol. You cannot choose two of the same gun, even if they have different attachements, but any combination of different guns is allowed. Hint: When this perk is used in conjunction with the Last Stand perk, you still pull out a pistol during the stand, even though you cannot use one regulary during your life. If this combination of perks is used, I believe you pull out an unsilenced M9 every time.
UAV Jammer
The UAV Jammer prevents you from showing up on enemy radar, which is best used with silenced guns or in Search and Destroy matches, where stealth is a huge factor in whether your team wins of loses.
Sonic Boom
Best used with the Frag x 3 or RPG-7 Tier 1 perk, this increases the damage and splash area [fact check?] inflicted by explosives like: frag grenades, RPGs, grenade launchers, claymores, and C4s.
Tier 3 Perks

Extreme Conditioning
One of the more useless perks avaiable in Call of Duty, this perk allows you to run for double the distance you would normally. Maybe you could use it in your first couple days playing to achieve the Marathon achievement and get some bonus xp, but after that, the other perks in this section are much for valuable to the player.
Steady Aim A perk best used in classes with LMG's, it increases accuracy while 'firing from the hip' I.E. without aiming down the sights. This is a good perk for someone who isn't quick enough to aim down the sights when surprised by an enemy, but after your aim improves with game time, it's advised that you use a different perk from this tier, as it will aid in your game more.
Last Stand The last stand perk allows you to pull out your pistol in a 'last stand' before you die. This stand takes place when you would normally be dead, I.E. you still not take the stand when you would normally be playing withouth this perk. ANY damamge received while in the last stand will kill the player, and the person with the final blow will be awarded the kill, not the one who does the damage that initiates the last stand. Although not on the same level as martyrdom, this perk is sometimes banned on servers, and you may be flamed for using it. In my personal opinion, you still have to aim and fire, so this perk deserves no negative words, but meh. I'm not the majority of the player base.
Martyrdom This perk drops a primed grenade at your feet when you die, hopefully taking down any nearby enemies with you. WARNING: Using this perk is huge flamebait, especially in HC mode. It is regarded at cheap to use, so avoid it unless you don't mind being verbally abused.
Iron Lungs This perk is an absolute necessity for any sniper class. This perk allows you to extend your held breath from the normal 4.5s to 9.5s, almost double the time to place incredibly accurate shots!
Dead Silence Dead Silence muffles your footsteps, making it less likely that you will alert an enemy that you are sneaking up on. In Tier 3, this is not one of the most useful perks, perhaps only being used in organized Search and Destroy matches, where a high level of stealth is needed.
Eavesdrop One of the least used perks in public servers, this allows you to hear enemy communications when you are near them. Because not many people use mics on public server, this is probably only used by highly organized clans. However, if you are a member of a highly organized clan, you probably know everything that I've written on this page, disagree with some of it, and wonder why you just wasted your time reading this whole article.