This document provides an overview of single sign-on (SSO) services using free and open source software. It discusses the SSO implementations at Universitas Indonesia and Peentar. At Universitas Indonesia, SSO is implemented using JASIG CAS to provide centralized authentication across various apps. User credentials are stored in OpenLDAP and additional attributes come from Drupal and REST services. Peentar uses ORY Hydra for SSO with an OAuth2 consent flow. Hydra is deployed with PostgreSQL and consent/identity apps are built on Rails. Both implementations provide secure centralized authentication while leveraging open source software.