Notes for Gmail now allows you to insert sticky notes directly onto your email threads in Chrome. The browser extension, currently in beta, allows you to annotate your emails as a whole (thread)—or individually. You can add as many sticky notes as you would like and they all appear at the top of the webpage for easy accessibility.
To download the application, go to this page and request an invitation. They add users everyday—I was added almost instantly after I applied.
Once you've downloaded the Chrome add-on, open Chrome and go to Tools -> Extensions. You're going to have to drag the download onto Chrome to make the add-on work. It should appear as so:
Now you can open up Gmail and use the sticky notes for whatever purpose. I've recently used it to remember which articles I should and shouldn't write for work. It works fantastic because I don't have to individually search through the email thread to find this information out—I can just look at the sticky note.
As of now the add on is only available for Chrome, but a Firefox one is in the works.