This plugin requires SparseBukkitAPI.
This plugin allows players to create portals that will link as long as the patterns match.
Simply create two or more frames that have an identical pattern rotated to all four sides.
Once you've built the frame, throw in a Nether Star (configurable).
If the frame is valid, the item will float to the middle and start creating the portal, during this time you can throw in modifiers.
Modifiers allow you to change or add functionality to a portal.
Any modifier can be disabled in the config.
Functionality Modifiers:
Particle Modifiers:
Torch:

Redstone:

Ender Pearl:

Dragon's Breath:

Dyes (Any Color/Colour):
Nether Wart:
Emerald:
Fire Charge:
Inner Block Modifiers:
Some modifiers are not compatible with others, if a modifier is incompatible it will not be picked up by the portal.
If there are more than two portals with the same frame, they will link to each other in a chain.
A -> B
B -> C
C -> A
Installation:
This plugin requires SparseBukkitAPI to function.
After installing SparseBukkitAPI, just put this plugin in the plugins folder.
This plugin is highly configurable, almost everything can be configured.
If you find any bugs, please report them along with any errors in the console caused by this plugin.
Known Bugs:
You cannot ride a horse/minecart/pig/etc. through a portal.
Rotated blocks must all be rotated the same way in the frame. (For example, if logs are pointing upwards in one part they must be pointing up in all parts.)
In the future I plan on making this open source, but for now it isn't.