Uses machine learning to identify files in misconfigured buckets across a large number of providers, including AWS, Azure, Digital Ocean, GCP, and Alibaba. Requires an account to get results but the free tier is pretty useful in itself.
API documentation: https://openbuckets.io/api-docs
You have to have a Bounty Hunter subscription or higher to use it, though. Good luck unsubscribing from their mailing list, even asking them personally to help doesn't work.
A search engine for open and unsecured Amazon S3 buckets.
Has a REST API. Have a key.