Boomerang is a plug-in that works with Chrome, Firefox, and Safari. It has a lot of great tidbits, but perhaps the most useful is the ability to "send later." You just write a message as you normally would and then use either a calendar picker or a text box ("next Monday") to schedule when it will send. You can also set a reminder to yourself if you don't hear back so you know when to follow up.

Get it here.