Free tool from Xangati which has an unique capability of real time monitoring. It’s a virtual appliance for one ESX(i) host, but you can deploy one xangati per host if you wish. You have a possibility to fully configure of What you See on the dashboard, even if the appliance comes up with 3 different modes already pre-configured.
One fo the new features announced in this release, which is compatible vSphere 5, is the datastore latency monitoring. In fact, It can detect and show a latency and IO stats from different paths to a datastore from a host.

His feature can also be coupled with VMware vCenter alarms.

Xangati for vSphere can be found for Free here.