Amazon Prime Photos allows unlimited photo storage (RAW included) with any $100/yr subscription to Amazon Prime. If you're already paying this for the other benefits, then it is free.
At the risk of "ruining" it for some users, Amazon doesn't appear to validate nor re-compress uploaded images. I uploaded some bits from /dev/random as a .png, and it counted towards photo storage, and I got the same bits back re-downloading the test file. So you don't even have to resort to stuffing metadata or steganography.
For the record, I don't exploit this loophole and only tested to satisfy my curiosity. At $60/yr. it's already very cheap for unlimited -- it's my tertiary backup so I'll accept the risk that "unlimited" is qualified -- generic file storage.
