Multiverse is a large and useful plugin allowing players to move between different worlds using portals, and have separate inventories in those different worlds.
Multiverse core - essential commands
Multiverse portals - allows players to teleport between worlds using any portal you like. You can use Worldedit to make the portals, but it is not essential
Multiverse inventories - allows you to specify in which worlds players have the same inventory - for example, on a server with a creative world and a survival world, you don't want players simply bringing supplies from creative into the survival world