Tornado is an open source version of the non-blocking and scalable web server and tools that power FriendFeed. The FriendFeed application is written using a web framework that looks a bit like web.py or Google's webapp, but with additional tools and optimizations to take advantage of the underlying non-blocking infrastructure.
The framework is distinct from most mainstream web server frameworks (and certainly most Python frameworks) because it is non-blocking and reasonably fast. Because it is non-blocking and uses epoll, it can handle thousands of simultaneous standing connections, which means it is ideal for real-time web services. We built the web server specifically to handle FriendFeed's real-time features — every active user of FriendFeed maintains an open connection to the FriendFeed servers.