What about http://cacti.net/ ? With Weathermap plugin it is quite good thing for performance monitoring. THold and Monitor plugins are for availability monitoring and alerting. It scales very good with Boost plugin, I had 3000+ hosts, 60000+ datasources with total poll time within one minute.

Many prefer Cacti to Zabbix because the latter is clunky and confusing compared to Cacti's simple straightforward approach with plugins.

But I am actually wondering, how alive is Cacti now, because its releases are constantly postponed, and forum is not as alive as it used to be.