Let's not talk about instances.
One of the things that interests me is approaches to architecting SaaS, Software as a Service. Recently I've been thinking about approaches to multi-tenancy; at a very high, "I wonder what... would do", kind of level - nothing deep, nothing tested (yet).
My thoughts started with the fact that everything is nested in different levels of containers. Each line of our own code runs within some context that we create within our application code, that in turn runs inside a container, this might be the OS or it may Microsoft .Net, the PHP runtime or a J2EE or Servlet container, which in turn runs inside the OS, which in turn runs inside a machine, which in turn runs inside a data center...