Back in September, Google launched Sidewiki. When you're using the Google Toolbar, you can access Sidewiki for any web page (that's been annotated with the tool). You'll see comments and notes about the site, as chosen and ranked by Google.
Now, Google is updating the API for Sidewiki. With the update, you can now view new entries created for a Sidewiki page via RSS.
If you're interested in developing with the Google Sidewiki API, check out the documentation here.