What I like at SezWho is the approach to leave your site comments untouched and search engine friendly.

I recently played around with Disqus and Intense Debate and I like the whole idea.
http://www.intensedebate.com/users/57981
http://disqus.com/people/Infopirat/

Other things that are maybe similiar but decentral:
http://www.cocomment.com/

and last but not least this here, which is the "Semantically-Interlinked Online Communities Initative":
http://sioc-project.org/