IPKarma is a new karma system to help track users who comment on your blog. The twist is it doesn't just track them for you, but also for any other blog owner who has the plug-in installed.
It tracks each user by their IP address (not their username or e-mail address) and automatically creates a user profile for them when they leave a comment. Similar to SezWho (coverage), other users can then click on the profile of a user and rank up or down their profile to denote whether or not they've found that user's contribution worthwhile, although there's no way to vote up or down on a specific comment. Each IPKarma profile also tracks all the comments you've left on integrated enhanced blogs, letting other users track down and read what you've written, along with any other commenting context right from your profile page.
There are a few caveats to the service's ease of use. Users can't opt out of having a profile created for them, and in order to "claim" their profile, they must receive a plus-10 rating, which means nine or more users must have voted a profile up.
Also, tracking things via IP can get a little tricky if your users are interacting with your site from different locations like home, the office, or while traveling. A system that uses e-mail addresses (which incidentally are required for claiming a profile) would be a much more foolproof way to track interaction, although I like the idea of taking some of the login responsibility away from the user in the first place.
To give the service a spin, you can check it out over at the IPKarma blog.