Google Sidewiki is an extension for the Google Toolbar allowing you to post annotations on a web site. Next time some other user with Sidewiki enters the site, your annotations will be available for reading. Is something like a comment… somehow… because it’s not within the natural flow of the website but out of it. I mean, it’s like you’re telling your friends “hey, this site is cool” or “this site sucks” but not really letting the owner of site be aware of what you think about it, unless of course, he visits his own site with Sidewiki enabled.
Here’s the video presentation by Google
All in all, I think it’s nice, as long as the feedback between the site and the user is not trimmed by Sidewiki. You can download the extension for the Google Toolbar (IE 6+ or FF 2+) from Google Sidewiki.