At this point, S3 IS the standard for that. And Google's and Microsoft's offering all mimic that API. This could happen for all other of their services.
Not just Google and MS, many smaller services too. Linode and Digital Ocean's object storage both use an S3-compatible API and a number of open source, self-hosted services do as well from what I have seen. The S3 API is the defacto standard at this point.