I truly believe that this is the best thing to happen to Gmail since, well, Gmail itself.
Boomerang is a third party plugin for the Firefox and Chrome web browsers which adds a ton of functionality and productivity enhancements to your Gmail account. If you’re a believer in the Getting Things Done (GTD) method, the Inbox Zero method, or both, Boomerang is for you. If you’re not a fan of either, Boomerang is still for you.
What Boomerang allows you to do, in a nutshell, is resend yourself emails from your inbox. For example, if you have an email from your boss which tells you to perform a task at a certain time, you can have Boomerang send that email from your inbox and schedule it to return at a specific time, serving as a reminder to perform that task. Got a meeting coming up and want a reminder half an hour before? Send that email via Boomerang and schedule it to return to your inbox a half hour before the meeting starts. Its arrival back in your inbox (and the resultant new mail alert), will do the job.