Prometric is one of the largest certification testing companies there is. Microsoft is the one giving the test away: http://www.microsoft.com/learning/en/us/offers/html5.aspx.

Yes, MS gives the exam for free, Prometric is just the testing center. I got my Server 2008 Active Directory configuring cert free through a similar offer.

I also got my A+ Cert from Prometric, its the main testing center (at least in the US)for these types of certs.