ZeroSSL – free 90-day certs via ACME, also has a web UI for cert management

Google Trust Services – free ACME certs, requires a Google account for registration

SSL.com Free DV SSL – offers free 90-day certs through ACME

I use acme.sh for certs on my personal server and was a little surprised when it started using ZeroSSL by default. Despite being more "corporate" I decided to roll with it and it's worked just fine.