Once it's installed, it couldn't be easier to use: Just open any email message and then click Add Note. You'll get a blank field into which you can type or paste your info. When you're done, you'll see the equivalent of a yellow sticky appended to the top of that email.

Mouse over the note and you'll see four options: Facebook, Twitter, Delete, and Edit.

The developers promise to add more features in the future (I'd like to see a date/time stamp), and probably a price tag as well: The product page notes that the service is "currently free," meaning it probably won't be once it leaves beta.

Which begs the question: How much would you pay for Notes for Gmail? I definitely think it's worth something, though I'm not sure I could see myself paying more than, say, $20 annually. Your thoughts?

While we're on the subject, check out .