sidewikiIt’s an idea that’s been done before: Let people write notes about websites they visit, and then share those notes with others. Diigo has been doing it for years.

[Update: Google’s reply to my email is included below.]

Google Sidewiki is different — it has the world’s biggest brand behind it. People who’ve never heard of Diigo will give Google a spin.

cdcSidewiki basically adds user comments to every page on the Internet, with Google sorting them for relevance.

For now, Sidewiki has no ads on it, and it doesn’t support the company’s Chrome browser. So it’s not clear what the business model is, or how it will fit into Google’s Chrome OS operating system.

“We currently have no plans to monetize Google Sidewiki,” Google spokesperson Eitan Encuya emailed me. “We hope that Google Sidewiki encourages more users to share useful information with others as they browse the internet and increase the engagement of users on the web.”

Google’s official blog post ignores P&L issues and says the hard technological problem they’ve cracked isn’t building and serving a wiki. It’s figuring out the relevance ranking system used to decide which entries to show, and making it scalable.

It’s too soon to tell if Sidewiki will be a hit, but it’s a safe bet that the usual bloggerati will jostle for position among the Sidewiki entries for Google’s homepage. Let’s take a look … hmm … Michael Gray … Danny Sullivan … Michael Arrington … I’m guessing Scoble hasn’t checked Techmeme yet this morning.