If you are using vSphere 5.5 Update 3 or vSphere 6 then NSX does not require a vSphere Enterprise Plus licence however if you are using vSphere 5.5 update 2 or earlier then a vSphere Enterprise Plus license is required.

This is because NSX is based on the vSphere Distributed Switch which is part of the Enterprise Plus licence feature set.

Reference: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2135310