I've just launched IPKarma, the Portable Karma service for Blog Commenters.
IPKarma lets commenters sign their comments with a (system-generated) avatar, linked to their IPKarma profile, which contains their commenting history and reputation. Commenters don't have to register or even know about IPKarma.
Installing IPKarma entails only displaying an avatar for each commenter, transparently fitting with the existing commenting framework. Bloggers don't lose control over comments, don't lose any search engine juice, and don't waste any blog real estate.
A loose analogy would be Gravatar + SezWho.
Hope you find this useful.