Caint.casa is a comment service. See below for a sample. Comments here will show up automatically, but can require approval by site owner's choice. Caint.casa is currently in use on this website, funnily enough, he's also the creator.
If you're looking to integrate caint.casa onto your own site, you can start that process here on the dashboard. There's a nice setup tutorial, and this is all really quite simple.
Caint.casa is currently a free service. It will remain free as long as I can keep it running on a tiny server. If anything ever changes with that, I'm sound, I'll give anyone using it advance notice of at least 60 days, and I'm working on a feature to let you export any data you might have on here anyway.
caint is licenced under the AGPL, and the source can be found here. Documentation for that is currently non-existent, but it should all be fairly straightforward if you've used ASP.net before. If you host an instance, please email me and let me know! I'm happy to help if you're having any issues setting up your own instance.