If you are looking for performance reports then they are captured in the vCenter server itself. You cn use vSphere PowerCLI to write your own customized reporting tool.
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Any free utility for vmware Vsphre report/
It depends on what data do you need.
Some are in vCenter, but there are a lot of 3th part tools (RVTools, Veeam, ...).
Or also a lot of scripts (as written before).
One of the best (free) ways to document a vSphere environment is to use the Virtualization EcoShell, which includes a way to generate reports and Visio diagrams. It is very easy to use and a great tool to keep in your toolbox for managing a VI3/vSphere environment.
Here is a link:
Here are some reporting scripts for both VI 3.5 and vSphere 4.0:
VMware vExpert 2009
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