MultiInv is a plugin that gives players a separate inventory for every different world running on the server. You can share inventories over chosen worlds and some players can be ignored entirely. With this power it is possible to have multiple worlds running on one server, but keeping them totally segregated from one another.
MultiInv can also separate health over certain worlds allowing for total separations of a world on a server.
Official URL : http://dev.bukkit.org/server-mods/multiinv/