Zenoss is an open source agentless monitoring tool. The main advantage of zenoss is that you don’t need to install any software on the client side for monitoring. Zenoss has the following two versions: Zenoss Core (this is free) Zenoss Enterprise. This is also called as Zenoss Resource Manager, which is a paid version.
Enterprise Networking Planet: "Open source network monitoring and management vendor Zenoss is out this week with a new version of its commercial Zenoss Enterprise platform. Zenoss Enterprise 3 is built on an open core model, with the core product based on the Zenoss Core open source project and including some proprietary bits on top."