The limitations are basically 8 vCPUs per VM, and the restricted API, which eg. prevents from managing the host in vCenter, and using most commercial VM based backup software.
There's however no restriction regarding commercial use, except for the written limitation for less then 100 hosts (you can see this on the registration page, below the license key).
Thank you for this answer to this question as I was asking precisely the same question yesterday
Could I add it will be helpful to have a specific URL for the download of the 6.7 free version?
Google is your friend: Download VMware vSphere Hypervisor for Free