Finding on host monitoring that works for OSX, Linux and Windows is tough. It is even tougher to do it without CPU and Memory intensive languages.
But, we are doing it with virgo. Virgo is a Rackspace project that is creating a tiny daemon using lua, luvit and C to do monitoring across all major OSes fast and securely. Learn how it is built, how you can hack on it, and what it can do.