Consider OpenAuth. It's what twitter uses, and it's gaining mindshare.
The principle behind it is that when you authenticate against a trusted third party, you are prompted by an auth page on the third party domain, so it's obvious to the user that you are logging in through them, hence the trust.
Facebook's third party login is very similar, where they show their login content in a lightbox or popup window.
Another common one is Kerberos http://en.wikipedia.org/wiki/Kerberos_(protocol)
