JanRain has written open source libraries for OpenID: http://www.janrain.com/openid-enabled http://www.janrain.com/openid-enabled Their free version of Engage

JanRain has written open source libraries for OpenID: http://www.janrain.com/openid-enabled http://www.janrain.com/openid-enabled Their free version of Engage is sufficient for basic authentication. As mentioned already, your authentication system will then rely on your site, Janrain's site, and also the 3rd party site that performs the actual authentication. If you just want to quickly get going, you can use their free version of Engage with the above caveats. If you want to not rely upon JanRain, then you can use one of the OpenID libraries out there. Integration is "relatively" simple, it should take 1 day in most cases. However, you may want more than OpenID (Hello, Facebook Connect and Twitter's OAuth). If that is the case, then you will have to add support for those sites as well. It would be nice if there is an open source version of Engage, but there doesn't appear to me from my searching.