Social networking sites are filled with comments about individual web pages, but that commentary isn't centralised. Pushnote (which counts Lifehacker perennial Stephen Fry amongst its backers) lets you leave comments directly on any web page for others to discover.<!—more—?
The service is integrated via plugins into Firefox, Chrome and IE (no sign of Safari or Opera yet). Once installed, you just click on the small speech balloon button Pushnote adds to leave comments on a particular page. If that page already has comments, the button will be green.
Pushnote will search your Twitter and Facebook friends lists to find other users, and let you invite other Facebook users to join the service. You can opt to post your comments to either service as well.
Pushnote is a free service, requires registration. It's a little rough around the edges right now -- a particular nuisance is that if you Alt-Tab midway through typing a comment to check something, the entire dialog disappears never to return. It won't necessarily take the place of sites offering their own comment facilities (ahem), but it could become an interesting way for people to add commentary on other uninteractive sites within a social framework.