There are some confusion with the packaging of the vCenter editions and I would like to clarify some of this with some of the links around VMware sites for easy understanding.
Essentials or Essentials Plus are bundled with vCenter Essentials while vSphere Standard, Enterprise and Enterprise Plus is often quote with vCenter Standard.
So what does this mean? With the different vCenter Edition, there are some hard limitations.
Also with vCenter Essentials, there is no linked mode and this is found in the vSphere 5 Documentation Center here. When planning a design, you need to know the limitations and the requirements before recommending the correct vCenter and vSphere Edition.
As for the features comparison of all the Editions of vSphere 5 from Essentials to Enterprise Plus, this is located in this KB.
Having summarize the above, customers who are on Essentials or Essentials Plus and like to have features like linked mode found in vCenter Standard, there is a feature limitation controlled in vCenter Essential. There is also vCenter Foundation edition which just limit to 3 hosts management.
vCenter Standard or vCenter Foundation cannot be purchased by users under vSphere Essentials and Essentials Plus licensing based on End User Licensing Agreement (EULA). The solution is to upgrade towards vSphere Standard or above with acceleration kits.
However there is a SKU for vSphere Essentials or vSphere Esential Plus for ROBO which allows vSphere Essentials/Essentials Plus to be managed by one vCenter Standard or Foundation. However do note of the limitation from vSphere Essentials and vSphere Essentials Plus still exist even with vCenter Standard or Foundation. Details can be found here. There is also a blog post on the most updated vSphere 5.5 for ROBO.
There is also another edition known as vCenter Desktop. However this is not available for purchased. It is automatic included in all VMware Horizon bundles.
There is also vCenter Foundation edition. This is mainly for customer who like to make manage up to 3 hosts of vSphere Standard and above. Unlike vCenter Essentials there is no 6 CPU limitation.
Update 14th May 2013
vRAM entitlement has been removed and updated with vSphere Essentials/Essentials Plus ROBO and update for various editions of vCenter.
Update 14th Mar 2014
Update information link for vSphere Essentials/Essentials Plus for ROBO.
Update 3rd Jul 2014
Added vSphere 5.5. for ROBO.
Update 12th Oct 2014
Explain vCenter Desktop.