Xangati from Xangati Inc. is unique for a few reasons, including how it combines virtual network analysis and vCenter performance data in one tool. Xangati deploys as a virtual appliance and allows users to "rewind" through the virtual infrastructure network communications and performance data to identify problems sources. This can help you prevent the problem in the future.
Xangati's virtual network analysis is a free vSphere tool that tells you what systems are communicating on the virtual network, what protocols they are using and how much bandwidth they are using (you won't find that information in vCenter or most other tools).
Features in Xangati's free edition match, for the most part, with the commercial edition. The free version's major limitation is that you can only analyze one host at a time, but the tool is still very powerful.