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AtteLynx

A local news site has such a system in their comment section. Judging by the scores, most people just end up using both for agree/disagree.
I became a registrar for .fi because I was tired of dealing with other registrars. I only provide domains for myself and my friends, but it's still easier, cheaper, and more secure than going through some third party.

That being said, I have no personal experience with other TLDs, but based on a bit of searching some do seem quite easy to become a registrar for.