Sprinting

Sprinting is a method of transportation that allows the player to move 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, or by holding forward and double tapping reverse. It is possible to jump across four blocks horizontally (although difficult), instead of the usual two blocks by sprinting, however the jump height is no higher than normal.

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 on the "sprinting state" the field of view also slightly changes (increases FOV by 10) to give the impression of increased speed.

History
Sprinting was mentioned at PAX 2011; the sprinting mechanic would expand the combat system and increase player mobility.

Jeb uploaded a video on September 2, 2011, featuring the difference between sprinting and regular walking.

Sprinting was implemented in Beta 1.8pre1 along with flying, and was officially added in Beta 1.8, the Adventure Update.

Trivia

 * When sprinting in water you get a very minor speed boost, but it isn't very noticeable. On the other hand, bobbing in water (holding down jump while swimming) gets you around faster.
 * Sprinting and holding down jump at the same time makes you go faster on land.
 * Ocelots, Iron Golems, Endermen and Baby Villagers are the only mobs that can sprint like the player.
 * Sprinting and jumping (spamming the "jump" key) while under a low (2-block high) ceiling causes a player to move at 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).

Bugs

 * When sprinting on transparent blocks (like repeaters, glass, leaves, farmland...), it is possible to sink through the block.
 * 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.
 * When sprinting on leaves, wool and slabs, the particles are grey. This is because the color is applied dynamically based on biome by the game (leaves, vines, lily pads, grass, ferns), or the block has a "damage" value changing the texture (slabs, wool).