After six years of not touching the registration page, I added "Login with Google" and "Login with Apple". (You can see exactly what that looks like here.)

Here are the results, after a week of testing:

1. Registration rate is up 13%.
2. 73% of new users register via Google/Apple.
3. Conversion rate from free to paying appears unrelated between "folks who sign up with username + password" and "folks who sign up via Google/Apple".

The entire project took me around five hours, start to finish — I used django-allauth, as the prospect of relying on a third-party vendor for registration seems fairly terrifying.

Probably the single highest-ROI thing I've done this year (and I've done a lot of high-ROI work this year!) Couldn't recommend it enough.