Have there been any apps/plug-ins that have truly changed your life?

Mine is Boomerang plug-in for Gmail. It's a plug-in that lets me write emails and schedule a later delivery time, but the most important function: it "boomerangs" emails at a later time that I designate. So previously, I would send an email and expect a response. If I didn't get a response, sometimes I would forget and things would fall off my radar.

But now I will send/reply to an email, schedule a time for the email to bounce back to the top of my inbox on a designated date if I don't receive a response, and then archive the email. This keeps my gmail inbox to a very minimal amount, usually 20 emails or less. But I rarely overlook things now!! Seriously has changed my life!!