* FusionAuth has a community edition which is free as long as you host it yourself and comply with the license ( https://fusionauth.io/license-faq#3 ). Free as in beer, at least. (Full disclosure, I work for FusionAuth.) Here's our SSO guide: https://fusionauth.io/docs/v1/tech/guides/single-sign-on
* If you want a SaaS solution, Cognito is an option, as is Azure AD B2C. They have good free tiers, as long as you don't want SAML integration (for Cognito, you're charged after 50 users).
* If you want to run your own, you could conceivably cobble something together on whatever tech stack you run. Ruby on rails, java/spring, django/python all have libraries which can be used to build OIDC servers. Then it's a question of setting up the sessions correctly. Here's an example I found via googling: https://tushartuteja.medium.com/a-simple-single-sign-on-sso-...
* As the parent comment mentioned, Keycloak is an option as well.