Chrome: When you don't take care of your emails as they come in, it's easy to lose them in the clutter of your inbox. Mailfred is a simple Chrome extension that allows you to schedule reminders so you don't forget to action your messages.
Mailfred works similarly to the previously mentioned Boomerang, but it's free to use since it uses an extension and a Google Script to get the job done. When you're in your inbox, you can schedule a reminder for a message at a specific time or a later date. The message then gets archived and you get a desktop notification from Chrome when it's time to check again.
It's a simple extension without a lot of features, but it does its job well.