Multi-site authentification "google like"

I want to create multiple services, which support an SSO (Single Sign-On) service.

To simplify, a google like services. With a single account you can use all services (and you log in once).

I found many ideas, but I want to have some opinions on how to design these services, and which technologies are the most adequate:

• many or one database ??
• creating an auth api ??
• sharing session ?
• using nosql databse or not ?
• duplicate user information foreach database ?
• separate users tables on a single database ?
• node js vs php !!
• ...