Nether portal

The Portal is part of the October 31st Halloween Update. It consists of a frame of Obsidian, four blocks wide, by five blocks tall. The corners of the frame are not required, and only serve for aesthetic purposes. Once the frame is constructed, the player then sets the space inside the frame on fire, using a flint and steel. This creates 6 portal blocks inside the frame, resembling a vortex. Here is a diagram of the process:



When the player stands in a portal block for a few seconds, the player is taken to The Nether. The player can step out of a portal before it completes its animation to abort the teleport. A portal will also be created in the place where you enter The Nether, which you can again enter to be returned to the normal world. Building multiple portals on Earth within a certain proximity will all lead to the same portal in The Nether, and the same might happen vice versa.

In the event of two portals (Portal 1 and Portal 2) on Earth linking to the same portal (Portal A) in The Nether, it will be necessary to build a second portal (Portal B) in The Nether. To find Portal B's location, travel from Portal A in the same direction but only 1/8th as far as you would to get from Portal 1 to Portal 2 on Earth. The direction can be determined by either aligning yourself in the proper direction as you enter the portal, or by using certain texture patterns on blocks to orient yourself. To determine the proper distance, rough estimation will work most of the time. To be more precise, you can count blocks or time a run from Portal 1 to Portal 2 on Earth, dividing the time by 8, and then running that amount of time from Portal A.

The purple portal blocks emit light, so passive mobs may crowd around active portals at night. Like Bedrock, portal blocks cannot be broken (although they can disappear); however, they can be placed with modding. These portal blocks will automatically disappear after a few seconds, or when a block is placed adjacent to it. The only exception is to place an obsidian block, and a portal piece next to that. The portal piece will disappear once the obsidian block is gone. However, portal blocks will not disappear in SMP.

Portals can deactivate if there is fire adjacent to it (started by the player or Ghasts). TNT or an exploding Creeper can also disable a portal, but cannot destroy the surrounding Obsidian. It is possible to "re-ignite" portals by setting the space inside the frame on fire once again.

Contrary to public belief, portals do not conserve momentum.

It has been confirmed that portals do not work in multiplayer yet; while they can be created, they can not teleport you. Server settings can be changed in order to visit The Nether in multiplayer.

Notch has mentioned it is extremely unlikely, but possible, for Ghasts to appear on Earth near a portal. 

Trivia

 * If you warp from world to world, the direction you are facing is maintained. (i.e if you were facing east on Earth and you warped to the Nether, you would still be facing east in the Nether)
 * An automatically generated portal may be built at a 90 degree angle to the one you entered. The direction you are facing is still maintained. (i.e. you may find yourself staring at the side of the portal)
 * If you place two portals on top of one another you will come out of the bottom one on Earth and the top one in The Nether.
 * If you run out of Flint and Steel and disable all of your portals while in The Nether, it is still possible (although difficult) to reactivate a portal by having a Ghast's fireball hit the portal instead of you. It has the opposite effect if the portal is already active however as if the portal is already active then the portal will be deactivated. This is unlikely and very difficult however.
 * When you look through an activated portal with water behind it, the water will not be visible.
 * Likewise, if you look through a portal with water in front of it, the portal blocks will be invisible.
 * Even when a portal is built with only 10 blocks of Obsidian (by leaving out the corners), the portal frame spawned on the other side will have the full 14 blocks.
 * If a portal created on one side (Earth/Nether) would exit into mid-air (on the Nether/Earth), the overworld side will have four additional blocks, creating a ledge to stand on.
 * It may be a glitch, but the player can strike portal blocks with tools, fists, and blocks, but objects will pass through them. This also explains why you can't put blocks and fire inside the portal frame while activated.
 * WARNING! Riding a minecart into a portal has some adverse effects. The player can go through the portal, but remains stuck in the ceiling of The Nether. Reloading that saved world again will drop the player into the Nether, possibly without a matching Nether-portal and possibly right into a lava lake. Riding a pig or a boat through a portal also gives this same effect. The object you rode will stay on the Earth side of the portal.
 * Zombie Pigmen grunts can be heard through portals.
 * If you use a portal while you are on fire, when you teleport to your destination the fire will be extinguished.
 * If you are in your inventory and holding an item while traveling through a portal the text and the loading bar will be gray.
 * If TNT is struck, then the player uses the portal to the nether, when they come back the TNT will explode.
 * A portal created in the Nether could create a matching portal in a Dungeon in the regular world.
 * Arrows get stuck on the portal instead of going through like you'd expect them to.
 * You cannot place a Portal beside another one. Once you ignite the frame of the 2nd portal, the first will disappear. You can, however, place portals 1 block away from each other, which creates an interesting effect between them.

Media

 * Here and here are two preparation videos with commentary, in which people craft portals according to Notch's specifications.
 * Here is a way to build a portal without needing a diamond pickaxe using lava and water.
 * On 29 October 2010 PC Gamer released this video, showing a portal being constructed and used.

Portal