The Email Game is a free tool for Gmail and Google Apps users that adds competitive elements to tackling your inbox. You get 3 minutes to process each email; just reply to or delete an email before the timer is up to accumulate points. If you fail to process an email before the timer runs out, you'll lose points, but there are options to pause or add more time for longer emails.
The Email Game has standard email features -- Send, Send Later, Label, Reply, Reply All, Forward, Archive, Delete -- and new ones. The Boomerang feature, for example, temporarily archives the email and returns it to your inbox at a later date.
By gamifying the increasingly tedious process of handling email, The Email Game incentivizes email efficiency and reinforces good practices. Check out a video of The Email Game in action below: