Boomerang GMail – allows you to schedule e-mails for later and setting up reminders.
Inbox Pause – literally installs a pause button on GMail so you can stop the influx of e-mails at any time. Useful for when you’re working with e-mail open but don’t want to be distracted by new messages or pestering clients.
FollowUp – allows you to remind yourself and others to follow-up on email messages. Simply add cc in firstname.lastname@example.org and you’ll be reminded in two days to follow up.
Rapportive – allows you to see a social profile of the person you’re e-mailing/replying to.