Chrome/Firefox: Boommerang Gmail, the Gmail add-on that can put a delay on your email sending, and "boommerang" messages back into your inbox at a better time later on, is now in open beta for anyone to try out.
The ability to delay and schedule sending is a nifty enough tool, if not something other plug-ins and tools already offer. Boommerang's trump card is its ability to re-deliver messages to your inbox whenever you want them, and have them come in however you'd like - starred, marked unread, labelled with Boommerang, or simply shoved back into your inbox.
The video below shows off some of the possible use cases for re-delivered email. I tend to think some people will use it to say "Oh, look, that email you sent was somehow delayed!", but I'm slightly evil that way.
Boommerang for Gmail is a free download for Chrome and Firefox, at least while in beta. Tell us how you can make use of delayed sending, or scheduled re-delivery, in the comments.