Boomerang

Boomerang is an extension that lets you schedule the sending of emails along with some other interesting features.

For example, the program can remind you about a particular email if, after a certain amount of time, you haven't received a response. Similarly, you have the option to store emails in a special folder until you need them.

All of this is integrated into Gmail's own interface and since it's from Google, instead of getting in your way, it fits perfectly. Boomerang is an excellent addition that doesn't take up any space on your computer and is a huge help if you regularly send and receive emails.