I've always thought that the way certain MMO games use cluster technology to support a massive single-instance world was pretty innovative. Second Life, for instance, uses a system where each 256m x 256m region of the world is mapped to a single server CPU core. (That's for "full" regions; the lighter-duty "homestead" regions get mapped four to a core.) EVE Online does something similar with its individual star systems, but more dynamically based on traffic in those systems. Certain heavily-trafficked systems, such as Jita and other trade hubs, get dedicated servers; other systems are shared among cluster nodes and "migrated" from node to node for load-balancing. There's even a means in place for alliances planning a major space battle to "reserve" dedicated nodes for the systems where the battle will be fought in advance. And now, they've even tied a second game into that same game world, DUST 514.