Our API directory now includes 30 authentication APIs. The newest is the Yubikey API. The most popular, in terms of mashups, is the Google Client Authentication API. We list 6 Google Client Authentication mashups. Below you'll find some more stats from the directory, including the entire list of authentication APIs.
Google announced in December of 2019 that it will soon require that third-party applications support OAuth 2.0 as a connection method for access to G Suite data. In June 2020 Google will begin denying access to users that attempt to login to less secure apps (LSAs).