|The people behind Project Orleans|
It may not sound particularly exciting, but Project Orleans can handle a large amount of data in ways other frameworks can only dream of. This is why it has been a particularly fond project at Microsoft/Xbox, which handles millions of players' online records, as well as thousands and thousands of games happening concurrently.
This is how you can see that your friend is playing War Games in Halo 4 while you're sitting in the dashboard, looking at your friends list. This is how you are able to send invites to join your party and chat, and likely has a hand in matchmaking on Halo 4 and perhaps even several other games -- and now it's open source to the public.