I don't actually think Cloudflare runs an ACME Certificate Authority. They just partner with LetsEncrypt? Edit: Looks like they don't run any CA, they just delegate out to a bunch of others https://developers.cloudflare.com/ssl/reference/certificate-...
ACME https://guide.actalis.com/ssl/activation/acme
Google https://pki.goog/
SSL.com https://www.ssl.com/blogs/sslcom-supports-acme-protocol-ssl-...
ZeroSSL https://zerossl.com/documentation/acme/
I don't actually think Cloudflare runs an ACME Certificate Authority. They just partner with LetsEncrypt? Edit: Looks like they don't run any CA, they just delegate out to a bunch of others https://developers.cloudflare.com/ssl/reference/certificate-...
Those are just providers that support the ACME protocol, not free certificate providers.
Google and ZeroSSL both provide free certificates via ACME. The links posted above have more details.