Unfortunately, I didn’t utilize many Free Teat Prep sites. I would attribute this to most of my “message board research” I did while preparing for the exam hammered home that you get what you pay for haha. So out of fear, in that regard, I decided to pay for prepcast.
I DID however utilize a lot of YouTube sample test where they ask the question, allow you time to answer, and then provide context as to why each answer was either correct or incorrect. Anything that provides you that post answer context I think will be beneficial (Free or Paid). I would look for free exams that provide that context, and not just allow you to test and check your score. Good luck in your pursuit of your PMP!