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@ndreas The comments-on-your-site-sphere is pretty packed. If you don't care about privacy, Disqus is probably the most well-known provider. If you don't mind paying for a service, Commento is a leaner alternative that does not track visitors or show them ads. And there are more options out there (some are self-hosted). AlternativeTo.net lists a handful.
I see others have recommended a couple of my plug-ins as well. Let me know if you need assistance setting them up.