Google Webmaster Tools adds new feature in Labs*, you can create a special page owner entry for Google SideWiki. It allows you to enter some information for each website individually, like having a comment board on every website (actually for every webpage). It allows several HTML tags, usual forum stuff like bold, italic, underline… url.
*Webmaster Tools Labs is a testing ground for experimental features that aren’t quite ready for primetime. They may change, break or disappear at any time.
You could enter SideWiki comment from various browser extensions, but only special owner entry will be always visible on top of other user comments.