OpenID it self, has a huge audience which cannot be easily ignored by any relying party web site.
Following two guides explain in detail, all you need to know to add OpenID relying party support and get it running...
1. Relying Party Developer Guide - II - Describes how to enable OpenID with Simple Registration and OpenID InfoCard login for websites.
2. Relying Party Developer Guide - III - Describes how to enable OpenID login with OpenID Attribute Exchange and OpenID Provider Authentication Policy Extension for websites.
If you have any questions, please join the identity-dev mailing list by sending a mail to identity-dev-request@wso2.org and post your questions there.