Boomerang for Gmail lets you schedule and send reminder emails right from your inbox.
Boomerang is my missing piece for gmail - absolutely the best thing that happened to my inbox in recent months!
You can use Boomerang to temporarily take messages out of your inbox until you actually need them. We'll bring it back to your inbox at the time you choose, marked unread, starred, or even at the top of your message list.
So you don’t forget why you needed it in the first place, you can now include a note with the message as you schedule it with Boomerang. When the message is brought back at the specified time, your note will be appended to the original thread.