Boomerang for Gmail
At a Glance
Note: This review addresses the beta version of the software. It has since left beta.
Boomerang for Gmail from Baydin lets you schedule e-mail to send later. It installs as an extension in Firefox or Chrome and adds a big "Send Later" button to the top of your compose window. Use it to schedule e-mail to go out in an hour, or tomorrow, or next month--anytime in the future, even if your computer is offline or shut down completely.
To use Boomerang, you need to give the service access to your Gmail account. The company says it only accesses headers, but technically could access entire messages. And in some circumstances even message headers can contain sensitive information. So you'll want to be careful using Boomerang with sensitive information or in regulated environments.
Boomerang is currently in beta and requires an invitation to try it out, but you can sign up for an invitation online or follow @baydinsoftware on Twitter and tweet that you want an invitation; the developers say they'll direct-message you an invitation code within a day.