Boomering is a Chrome and Firefox Extension that helps you schedule the emails you send and receive for a later time. It's currently in private beta, but we've got 2000 invites so you can give it a try.
It's hard to maintain a tidy inbox when you can't act on everything that comes in exactly when it comes in. Maybe you get reminders to pay your bills throughout the week but pay them all together on Sunday. Boomerang will let you schedule those emails to reappear in your inbox when you're reading for them. It also works for sent messages, so you can postpone delivery of, say, a happy birthday email that you remembered to send a few days early.
It works by placing a Boomerang drop-down menu in your Gmail message bar. From there you can choose how you want to schedule the message. Common options like in an hour, tomorrow and next week are readily available, but you can also choose a specific time. What's really nice about scheduling with specificity is that you don't have to entire an exact date and time stamp if you don't want to. You can enter things like "next Wednesday" or "Today at 5:00 PM" instead.
It's worth nothing that Boomerang only shows up when viewing or composing a single message. Currently it won't show up in mailbox view, so you can schedule multiple messages at one time. This is a beta, however, so maybe we'll see that feature in the future. Being a beta, expect some bugs. Currently in Chrome the Boomerang button may not show up in message view without a quick reload of the page. That's being fixed ASAP, but if you're a Chrome user don't panic if you don't see it — just reload.
To gain access to the beta, go to the Boomerang for Gmail download page and enter invite code lifehacker.