ImpressCMS OpenID integration was originally based on the Sakimura's excellent OpenID module for XOOPS. More information about this module can be found on the author's web site: http://www.sakimura.org/.  
OpenID Module enables users to login to Xoops and Xoopscube sites with their OpenID.
Once successfully authenticated to an OpenID Provider such as Yahoo!, AOL etc., users are asked whether s/he has an existing account at the site to link the OpenID to that account or create a new one.
Supports OpenID 2.0 and thus, XRDS, XRI, etc. Also, it supports sreg so if the OpenID Provider supports it, it can populate the Xoops users table as well.
Tested against Yahoo!, Linksafe, 2idi.