For example, suppose you get a sales inquiry from a potential customer. You could attach a note with details on when you replied, what you said, how you plan to follow up, and so on.

Likewise, if the boss emails you about an important project, you could embed related information inside that email rather than keeping it someplace separate. Heck, maybe you just need to remind yourself who the sender is, like if it's someone you met at a conference. The possibilities are fairly endless.

is a new browser plug-in that, in case the name didn't give it away, lets you add notes to Gmail. It's a simple but incredibly useful tool, and for the moment it's free.

It's also in private beta, though I was approved immediately after clicking Request an Invitation. Your mileage may vary.

For the moment, Notes for Gmail works only in Google Chrome, though a Firefox version is coming soon. Because it's not yet live in the Chrome Store, you'll have to install it manually -- which isn't complicated, but you'll definitely want to follow the instructions if you're not familiar with the process. (In a nutshell: Download the add-on, open Chrome's extension manager, then drag the add-on to it.)