Monitoring Dell/HP Servers Running ESXi (Free)

If you're running HP servers, use either the HP-specific build of ESXi or install the HP Management Agents Offline Bundle and offline tools bundle for ESXi after the fact. The bundles are also available here. This at least gives you some visibility into the system from the vSphere client perspective. It will detect drive and storage system health for locally-attached disks.

Couple this with an NMS solution (which would be helpful for the environment anyway), and that would be the solution.

You can access the CIM providers using something like Nagios/Opsview. There are modules in the Nagios Community Exchange to access these depending on what hardware you have.

We monitor our Dell ESXi servers using OpenManage Server Administrator. This will give you hardware info inside ESXi and setup an OpenManage node you can connect to for more hardware and array info.

Instructions are here: http://deinfotech.blogspot.com/2011/08/installing-dell-openmanage-server.html. They work for ESXi 4.1. ESXi 5.0 requires a newer version of the OpenManage VIB and Managed Node.