The "Super Overhang" world generator
hCraft has been designed to be a fully multiworld server from the grounds up.
Players can create worlds, switch between them at any time, backup them, restore them from backups, regenerate, resize, etc...
Every world has its own world generator, provider (determines the format from which the world is loaded/saved to) and physics threads (up to 24).
And just like the good ol' classic days, worlds can have a finite size and have that awesome border (although they can be infinite as well).