This is a transportation plugin for Bukkit which allows for quick teleportation between any number of locations or worlds. Users build portals using 7 obsidian, a door and a redstone torch, then are able to link them to wherever they choose using a few simple commands.
The youtube video below shows how the plugin works:
For in-game help, you can also type /portal help.
More information and downloads are available on the BukkitDev Page