Recently Google has introduced Sidewiki. Once you download the sidewiki extension, while surfing any page, even google search results, you can add a sidewiki "annotation" to that page.
How it works, is the Sidewiki icon will appear in your Google toolbar, once you've downloaded it. On any page, you simply click on the Sidewiki icon in your toolbar, and a small window will pop out from the left side of your screen. You add your comment to that and you can either leave it popped out, or then close it.
Now the previously white Sidewiki icon in your Google toolbar will turn yellow to indicate that there is a comment.
If while surfing, you see your Sidewiki icon yellow, then you know that someone has annotated this page. You can click on the icon and read their annotations.
How it works, is the Sidewiki icon will appear in your Google toolbar, once you've downloaded it. On any page, you simply click on the Sidewiki icon in your toolbar, and a small window will pop out from the left side of your screen. You add your comment to that and you can either leave it popped out, or then close it.
Now the previously white Sidewiki icon in your Google toolbar will turn yellow to indicate that there is a comment.
If while surfing, you see your Sidewiki icon yellow, then you know that someone has annotated this page. You can click on the icon and read their annotations.