Hanselman does a whilrwind tour of the various popular web2.0 systems that have C# or .NET APIs.
In my book, this is all interop.
What makes it all possible? First: cool, interesting, programmable services. I don't have high value scenarios for programming Picasa, or Twitter, but apparently somebody does. And ... Second, .NET is interoperable. It works with XML , with JSON, with HTTP and so on. For those of you who have been using .NET for a while, this is no surprise. In fact it sounds inane to you, that I keep saying ".NET is interoperable" ; But listen, it wasn't that long ago....
All the connections we're seeing these days are beautiful.