Sprinting

Sprinting is a feature in Beta 1.8, which allows the player to run faster, around 5.6 meters/second. This is about 30% faster than the normal walking speed of around 4.3 m/s. The player sprints by double tapping the "Forward" key ( by default) and then holding it. It is possible to jump across three blocks horizontally (instead of the usual two blocks) by sprinting, however the jump height is no higher than normal. Whilst sprinting, the player is able to clear gaps of one block without jumping.

Sprinting is unlimited as long as the Food Bar is above 30%, but it also depletes the bar at a rate of 1/2 a food unit every 40 meters, or 7 seconds, as long as the foodSaturationLevel is at 0. When a player sprints, particles come from the player's feet corresponding to the block type they are running on. When a player attacks any mobs (excluding squid) while sprinting, the mob suffers a large knock back.

Sprinting stops when you collide with a solid block, or perform a sneaking action such as blocking. You do not stop sprinting when you run into a mob. Attacking a mob stops your sprint, but breaking a block doesn't. In addition, the player isn't set on fire when you run through up to four consecutive blocks of fire (the fifth block does ignite the player). It is possible to make turns with the "Left" and "Right" keys (by default, and  respectively) while sprinting without slowing down, thus altering the direction of where you go.

After switching in the "sprinting state" the field of view also slightly changes (increases FOV by 20) and the camera moves back to give the player more view of the terrain.

Trivia
Currently the fastest possible movement in Minecraft can occur when the player sprints and jumps while in a 2-block high space/tunnel on top of ice. This causes the player to move at about 16 blocks per second. Contrasting that is the slowest possible movement in Minecraft, which can occur while the player is blocking and moving forward while going through cobwebs on top of soulsand that is on top of ice. This causes the player to move at about 1/32 blocks a second, or 1/2 pixel a second on default texture pack resolution versus the 16 block per second with the previous trick.

Bugs

 * Sprinting and jumping while in a 2 block high tunnel or house causes a speed almost twice as high as regular sprinting speed. This is rather inefficient, however, as this can easily drain food at a rate of about 1 unit per second, but it proves an excellent form of transport when playing on peaceful, or if one has a large amount of food.
 * Doing the same as above, except on top of Ice allows the player to move at an astonishing 16 blocks per second. That is twice as fast as a full speed minecart on powered rails.
 * Doing the same as above but with trapdoors just above the ice, one can travel 1000 blocks in 54 seconds (18.518 blocks per second).
 * When sprinting on unusual level surfaces (like repeaters) it is possible to sink 1 block into the ground.
 * When sprinting, you can run over one block wide gaps. It is unknown whether or not this is a bug.
 * Water does not stop you from sprinting, and the normal particles generated from sprinting are shown (as water block particles), but you don't move noticeably faster than normal while in the water.
 * A strange minecart boost can be accomplished by sprinting and punching the cart and right-clicking the cart before it moves away. This can be used to go up an around 15 block high rail slope. It is not known if this is a bug or not.
 * It is also possible to sprint by double tapping the shift key while holding down the forward button.
 * In the Beta 1.8.1 update, if the player attempts to sprint while flying in Creative Mode, the FOV will change change/increase slightly, but the player will not move any noticeable amount faster than the forward flying speed.