ClipFlair Forums. Create discussion topics and get answers
May be useful: http://annotateit.org/ it is designed so that one can use their own authentication and annotation store (https://github.com/okfn/annotator/wiki/Getting-Started - also has plugins for Wordpress there) added suggestion to MonoX team at: http://www.mono-software.com/Mono/Pages/Discussion/dtopic/pTwzqCo9402BoqFUAODB9g/Annotations-for-pages-and-blog-posts/
This is also interesting for image annotations: http://annotorious.github.io/