Multiverse

Multiverse is a plugin that can generate different worlds for servers. It is created by fernferret and its team.

Bukkit     Tutorial

The most amazing part of it is Multiverse Portals. It can make portals to other places and worlds. But WorldEdit5 is required.

Core
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Portals
Multiverse-Portals  provide portals between worlds, similar to vanilla portals, except the destination can go ANYWHERE, as opposed to just the nether. Note that the Nether portal blocks (the purple things that make the portals make the weird sound)  will not fill these portals, but the portals can have water or lava placed into them for a more authentic experience. If you're seeking a multi-world authentic experience, you should check outMultiverse-NetherPortals.

A portal can be constructed out of ANYTHING. Yes. Anything. Even Nothing. Step 1 of creating a Multiverse Portal is to create the 'Portal Frame'. Here are a few examples: