Is there a multi-world bukkit-plugin with per-world(-group) inventory?

In my opinion you should get Multiverse-core and Multiverse-Inventoriess and Multiverse-Portals

It's an awesome plugin set to get you more worlds and you don't need to go into the config files you just type /mv create and in the creative world you can do /mvm set gamemode creative

Hope this helped! :)

Yes, there is! You can use Default Commands, which brings multi-world support with a join-command to get to the other worlds and a really huge per-world configuration-file. You can set whether the world should be a creative-world or not, or if there should be a separate inventory for a world.

If this plugin is too extensive for your needs, you may use MultiInv which lets you also have separate inventories.