The standard open source AIM product in academia is Shibboleth (http://shibboleth.net/). It's a mature and powerful product that's in use all over the world. It normally uses SAML but there's an OpenID extension.
https://wiki.shibboleth.net/confluence/display/SHIB2/IdP+OpenID+Configuration
However I'll warn you, configuring can be a chore ime. I've not used the OpenID extension myself.