Boomerang is a must-have alternate to the ever-increasing creep of Gmail "inbox pollution" from ill-timed emails, or to the crowding (and frequent loss) of highly-important emails in my GMail inbox by the torrents of relatively unimportant mail we all typically sort through each day. I should also add that Boomerang is vastly superior to every other system I've tested thus far, whether GMail or browser based, that promised (using one method or another), to help reign in out of control inbox noise but ultimately fell short.
Boomerang was of instant and high-utility for me and having used it for several months now have confirmed it so helpful that I felt compelled to pen a brief review and make others aware of this new service. I'm not sure whether it's considered an application "app", an extension, or plugin, but I do know that Boomerang is super-lightweight (with barely any measurable resource usage of my system) and has quickly become the most effective productivity tool I use for both personal and business email and task management.
I highly recommend their "Personal" service plan, and just now upgraded myself to their "Professional" plan in order to get access to the mobile version of Boomerang, which is available only to subscribers of their (presently-priced at $14.95) top of the line "Professional" offering. That said, my proverbial jury has just gone out on the question of whether or not the Pro offering is of relative value to the Personal service level (I'll check back in a few weeks with my findings on that question); but again, being able to effortlessly reschedule the handling of various GMail emails until such time as is ideal for me to re-receive and deal with them is an enormous boon to my personal productivity. In addition, the ability to schedule the exact delivery date and time of my emails (using natural language such as "in 1 hour", etc) each now invaluable capabilities I use throughout my typical day.
I know they have a free test version at Boomerang; check it out.
BTW: The above sounds really salesy, so I want to make clear that I am not an employee, stakeholder, etc., nor do I even know anyone at the company...I'm just a raving evangelist for their product!