Integrating Authentication For Different Web Applications

What is the best way to share authentication information between two unrelated applications. Is there a standard for this?

For example, if I use MoinMoin and vBulletin on a same website, is it possi开发者_C百科ble for both systems to share the same authentication information (i.e. so, a user will only need to signup for 1 to have a common login for both)?

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Possible yes. Worth the effort? I'm not sure. I would look at one of the open authentication systems such as OpenID to get this kind of thing to work.