I have been using this and love it. One of the things I use it for is to remind myself in the future to do a specific thing at a specific time (I have Gmail on my iPhone, so it pings me when I get the email), but in a way that's phrased as a request from a past self (e.g. "Hi future me, Do this thing! It'll be awesome! Cheers, past me") so it'll feel like I'm breaking a social commitment instead of just ignoring a reminder.

I added Boomerang based on your recommendation. I already use Mailbox for iPhone to schedule emails to reappear at some given time. I've used Boomerang primarily to delay the sending of an e-mail ("Send Later") and to schedule emails to reappear in my inbox if I don't get a reply. I find "Send Later" particularly useful, since it removes the rationalization that "I'll compose this e-mail later, since I can't send it out until later".

Great recommendation. I actually came back to this page with the specific purpose of recommending Boomerang, after trying it for a coupe of weeks on the basis of Qiaochu Yuan's endorsement, and I'm happy to see it already mentioned.