Let's Encrypt
7.6#3 in public key infrastructure softwareFree planFree planWebAPILinuxmacOS
VisitNo prices published.
Let's Encrypt serves people and organizations worldwide who need certificates for domain-named web, mail, or FTP servers. It suits users able to manage certificates through ACME clients and consult documentation or community forums when needed.
You want what its maker publishes above — check its page for the details.