Libevent 2.0.x is the new version of Libevent from the Tor Project. Libevent is a software library whose purpose is to provide consistent fast interfaces to various operating systems' mutually incompatible fast networking facilities. gives applications two basic interfaces to these networking layers: a low-level interface where the application is notified when an operation (like a network read or write) is ready to begin, and a higher-level interface where Libevent itself manages network operations and the application is notified when the network operations are completed. It's amazing how complicated things can become when you set out to do a simple task pretty well.
More info: Google Open Source blog