The libevent API provides a mechanism to execute a callback function when a specific event occurs on a file descriptor or after a timeout has been reached. Furthermore, libevent also support callbacks due to signals or regular timeouts.
libevent is meant to replace the event loop found in event driven network servers. An application just needs to call event_dispatch() and then add or remove events dynamically without having to change the event loop.
Currently, libevent supports /dev/poll, kqueue(2), event ports, select(2), poll(2) and epoll(4). The internal event mechanism is completely independent of the exposed event API, and a simple update of libevent can provide new functionality without having to redesign the applications. As a result, Libevent allows for portable application development and provides the most scalable event notification mechanism available on an operating system. Libevent can also be used for multi-threaded applications; see Steven Grimm's explanation. Libevent should compile on Linux, *BSD, Mac OS X, Solaris and Windows.
More information about event notification mechanisms for network servers can be found on Dan Kegel's "The C10K problem" web page. Another library that abstracts asynchronous event notification is liboop.
A mailing list for libevent is now available. Libevent is maintained by Nick Mathewson and Niels Provos.
NOTE: Specs from fedoraproject devel cvs.