This article by Wilco van Bragt describes what the free product actually offers and how useful these free versions are, it will just mention and compare the available features.
When considering a free hypervisor you should always test the product to see if the offered features offer the functionality you would like to have.
There are big differences in the features offered between the free hypervisor editions by VMware, Citrix and Microsoft. The VMware vSphere Hypervisor offers a very limited feature set and is mostly only usable for evaluating purposes or running test environments. On the other hand Microsoft is offering the most features in their hypervisor. Actually the options offered are suitable for many infrastructures and can be used to deploy a production environment. The only thing that is really missing in Hyper-V is a full central management in the free edition. Citrix is somewhere in the middle with offering functionalities, but offers central management for the free edition. In my opinion central management is key for a hypervisor infrastructure.
As mentioned in the beginning of the article I'm not going to discuss the quality, performance, usability, ease of operation and so on of the hypervisors, and just compare them on the functionalities offered in the free edition. With that on mind Microsoft will be the winner purely on the functionalities and if central management is an import feature Citrix is the way to go. VMware is far behind and personally I think the vSphere Hypervisor is just a loss leader for purchasing the commercial editions.
Always keep in mind when you start with a free hypervisor that overtime you may need to have additional features. VMware offers the most features in their product while Microsoft does not offer any additional features in their product. Citrix again is in the middle with the feature set. But just should also think about the necessity of features as I described earlier in this article series The need for advanced features within server virtualization.