In Modern Warfare 2 it seems that people have taken the game further than ever before in that they have come up with new tactics and new strategies for killing their opponents and winning games (this has been vastly helped by the developers, who have added things such as the Riot Shield and the 'Lightweight' class into the game).
By far the most annoying and the most difficult to deal with of all of these 'new' strategies is the tactical knifer.
In Modern Warfare 2 it is possible to equip a tactical knife with your secondary weapon (as long as it is a handgun, such as the USP or the Magnum).
This means that your melee/knife attack is far more effective due to the speed that you can perform it at and because of how quickly you can repeatedly do it.
Combine this with perks such as 'Marathon' (which gives you infinite sprint, enabling you to get around the map much quicker), 'Lightweight' (which makes you character much lighter, making him move a lot faster) and 'Comando' (which increases the range of your melee attacks) and you have a devastating combination.
This is actually quite a popular setup in Modern Warfare 2, yet I have yet to see an effective strategy for countering these players online yet.
Most people try to cover their tracks using claymores, but these people move so fast that the explosion usually misses them.
My strategy for dealing with these people is simple, but you'll first need to get yourself a decent headset that gives you surround sound.
Because most of these players don't use the 'Ninja' perk you can often hear them coming.
As soon as you hear them, whip out your shotgun and take them down in one easy shot.