In order to maintain sites in the cloud, Yahoo uses Traffic Server, a piece of software initially acquired via Inktomi, to support this massive amount of traffic. Traffic Server enables the session management, authentication, configuration management, load balancing, and routing for an entire cloud computing stack. Yahoo says that with the open source version of Traffic Server, organizations can benefit from access to cached online content.
http://www.techcrunch.com/2009/11/02/yahoo-open-sources-traffic-server/