Monster World, with more than 1.1 million daily active users (DAU), is a Ruby on Rails application with a MySQL/Redis backend that’s hosted on Amazon EC2. Brain Buddies (200,000+ DAU) and Bubble Island (another 1 million DAU) are both run on PHP with MySQL at Slicehost. Its newest game, Happy Hospital (already over 200,000 DAU), is a RoR app with a Redis backend hosted on dedicated machines at Hetzner.