Single Sign-On solutions are probably your best bet. The following are all OpenID providers (though some services, like Janrain, PingIdentity, onelogin, etc. support multiple protocols, and Facebook Connect also has its own API):
- Facebook Connect
Janrain is probably the most comprehensive service, followed by PingIdentity and onelogin. All 3 offer turnkey solutions for enterprise businesses. With Google and Facebook Connect, it's more of a integrate yourself solution for developers.
There are also these alternative SSO providers using different protocols:
- OpenAthens (for government and educational institutions only, I think)
But most of these will probably be harder to implement.