We run WordPress multi-site on WP.com to power several hundred million user accounts. So whenever you visit a domain hosted by us, you're hitting 99% core WordPress, plus some custom plugins and code we run to make it multi-datacenter and super-secure.

Calypso is our JS dashboard that lets you manage all your sites in one place, plus get cool WP.com features like stats and notifications. It's just a REST API client, just like the iOS and Android apps.