Reactor (deprecated in favor of Tokio)
A high performance, cross platform library that makes event-driven network programming easy.
Reactor is a thin wrapper around mio whose primary goal is to unify the event loops of various networked libraries. The goal is to be able to handle the network event management of a web server and a database client (and connection pool) in the same event_loop.
As mentioned, it is a high performance, low overhead event manager, it allocates very little at runtime, (boxed handlers at connection time), presently, a completely serial message passing benchmark over TCP can run 50,000 round-trips per second on a local machine, that's about 20 microseconds seconds per round-trip.
Reator Contexts are the core abstraction around a socket or datagram receiver. The primary event handler features a mailbox style interface, which matches across all of the events which might effect a socket.
API Docs including a simple example can be found here