I'm just starting a new side project, and have looked at Keycloak and fusionauth. Fusionauth has a free offering (ie, self hosted), and takes a minute to install and run. I like it a lot and am going forward with it for testing.

You won't see a login with fusionauth button, you'll just see a login button on my website, which might give you Google, github,LinkedIn logins etc depending on what I choose.

Depending on what set of features you need, it may make sense to also have a look at SuperTokens.io. Probably not as mature but we're open source and stronger with customization of the end user experience (frontend stuff), support and ease of implemenetation.

> stronger with customization of the end user experience

I might say "a different approach" :) I appreciate SuperToken's SPA first stance, but FusionAuth is pretty darn customizable too: https://fusionauth.io/docs/v1/tech/themes/

FusionAuth also has advanced registration forms and we are working on a lot of really awesome improvements around sign-in and sign-up workflows.

Stay tuned for some major updates in the coming weeks! ;)

Sure, just like you won't see sites with a 'sign in with Auth0' button. Both are service providers.

As a sibling comment mentioned, FusionAuth has a free offering if you self host. https://fusionauth.io/download/