The passed pawn

The value of a pawn can vary during the course of a game. The nearer a pawn gets to the queening square, the more dangerous it becomes. But the way to the 8th rank must first be cleared of opposing pawns.
The value of a passed pawn is of course higher. This value becomes even greater in the endgame, when there are fewer pieces about to endanger our pawn, and thus more chances of promoting our pawn.

In general, having a passed pawn can be considered an advantage. But how strong it is depends on other factors, such as the position of the pieces. 
The player with a passed pawn must aim to support it with all his forces and advance it. One can either try to promote the passed pawn, or, if the opposing forces manage to stop our pawn, we can switch our attack to other targets. The passed pawn may be sacrificed in order to deflect the opposing pieces. We have seen such a procedure in pawn endings. 
The side which is playing against a passed pawn must generally try to neutralize the passed pawn in good time. A good method is the blockade. In the middlegame the best way to stop the passed pawn is to blockade it with a bishop or a knight.

"The passed pawn is a criminal who must be kept under lock and key." - Aron Nimzovich

"the best way to stop the passed pawn is to blockade it with a bishop or a knight"
++ A knight or a king are better to block a passed pawn than a bishop.

Depends on the position. In the middlegame the king can be very vulnerable while blocking the pawn. But in general, the knight and the bishop are the best (especially the knight).

Since the Knight moves directly from square to square (unlike the Queen, Rook or Bishop which move along straight lines) the Knight is usually considered to be the ideal blockader, better suited for the task than any other piece.

Another point is that the Knight is the least valuable (point-wise, 3 points) of the pieces, so it is the hardest to drive away. You could drive away a Rook, Queen or King by poking it with a Knight or Bishop. Not so a Knight.

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another aspect of knights and pawns is, the peices essentially move and capture opposite of each other. the pawn once blockaded by the knight cannot forcefully remove the knight by its own power. the knight must be inacted upon by an outside force.

this also shows the superiority of the knight over the pawn. the knight controls the interaction between the two pieces.

although, two advanced passed pawns are generally superior to a lone knight even though it is only 2 points versus 3.

Kasparov gave an interesting discussion once on the subjects of force, space, and time and how they interact with each other to change values in a given position.

A knight as a blocker attacks the two squares where pawns can protect the passed pawn.
A king as a blocker attacks the passed pawn.
A bishop as a blocker attacks neither the passed pawn, nor the pawns that can protect it.