libivykis is a thin wrapper over various OS'es implementation of I/O readiness notification facilities (such as poll(2), kqueue(2)) and is mainly intended for writing portable high-performance network servers.
A few years ago (ok a LOT) I had written a VERY simple network monitoring system for work. It primarily gathers syslog events, and does a basic TCP connect to check on machine state. This uses MSDE as it's engine for events.