Dissenter is a Gab-powered browser-extension which allows you to comment on any video, any post, any picture on the internet. The only thing you have to do is to install it on your browser (directions varies from browser to browser, included on the download page) and make a Gab-account. Gab is a free-speech social media platform.
Get it now and start thinking for yourself at https://dissenter.com
EDIT: Dissenter can not let you comment on separate media quite yet. The extension is still in alpha-testing phase.