Bukkit

6x 1x 1x

Portals are 3x3, and consist of 6 pieces of obsidian, a door, and a Redstone torch for the center. From above, the portal looks like this:

n o n
o R o
n D n

o is a stack of two Obsidian blocks

D is any door

R is a Redstone torch.

n can be any block or air. It does not matter.

1. Construct an Obsidian frame like this:

2. After that place a Door in front:

3. Place a Redstone Torch inside the portal:

 It should tell you that you have created a portal, and fill up with water.

5. After this, just give the portal a name with /portal name <name>, and give it the name of another portal for the destination with /portal warp <destination name> and you're set.

As long as the destination portal exists, walking into it will take you straight there. For help in-game, type /portal help.

Yes, this does work across multiple worlds.

Using /portal help will list all of the commands in-game. The important commands are /portal name, /portal warp, /portal list, and /portal hide.

Here is a video explaining exactly how to make them, provided by MinecraftZero. More videos can be found on YouTube.