I LOVE Boomerang. Out of all the productivity hacks, this has got to be in my top 3. Why? Boomerang is like an email magic trick. The plugin lets you schedule emails to be sent whenever you'd want, snooze emails, and a bunch of other useful tools to ease your inbox woes. Best of all, it's free for up to 15 scheduled messages.
I'm all about work/life balance and this helps me attempt that lofty goal by letting me respond to an email, and then schedule it for the next morning or afternoon. This way, the people I communicate with see me responding during working hours and sets the expectation that 9am-6pm is my typical response window. How's that for a productivity hack?