Google Sidewiki is a wiki function against with the websites. In a general terms, that is similar with writing a comment agianst with a website.

Today, Google listed some greatest entries in sidewiki and the information about the new features of sidewiki in API in their offical blog. That is interesting but if you want to read the sidewiki directly, I suggest you install Google Toolbar, you can get sidewiki from the website URL directly.

Reference Link:
Google Offical Blog