You can use SSO software to treat all of the sites as part of a single 'authentication' realm. That way you log in once and then each site pings back to your SSO software to get the token or authentication confirmation.
One example of SSO Software is Shibboleth. It acts as a central gatekeeper and you can set up Service Providers on your server to protect your site and send it to the Identity Provider. Some details on using Shibboleth with Sitecore are here:
Owin/SAML federated authentication
Another example would be using something like Owin for federated authentication. Bas Litjen has a module for federated authentication which can consume authentication tokens and log users in:
More details are also available on this SSE post: Does Sitecore support SAML?
Active Directory module
Another option is using Sitecore's active directory module for authentication. There are some details on using it for SSO in this SSE post: Active Directory and SSO in Sitecore 8.1
Gigya (credit to @Mohit Dharmadhikari)
Gigya is one of the top Identity as a service (IDaaS) platform. It has loads of other offering as well. There's a Gigya Connector for Sitecore available on the Sitecore Marketplace.