SalesForce is probably by far one of the best ones out there. Of course it's not free, but the pricing starts as low as $5/month that should still help serve your needs.
What a great question. It scares me every time I release a new update on an MMO that I work on with only 2,000 concurrent users, let alone changing an algorithm that affects millions of peoples financial.
I would love to hear if anyone has insight on this as well.