Funny that no one has mentioned E2V. It's the only one that hit the 'I wanted to do that' criteria for me. I think the lesson is that you need to work hard to educate your user to get it to catch on. Think of specific killer use cases. I would recommend going to the nearest business school during interview season and posting a bunch of flyers that say something like "We'll call you when you get the email that's going to change your life". Maybe target med school applicants similarly, anyone who's checking their email every 5 minutes waiting for the big one. Don't target developers, they already know how to forward emails to text message. This is also the easiest to monetize without charging. If people give you 5 email addresses, the domains may well give you insight into what they care about most. Targeting advertising rocket fuel (applying to Goldman, are we?).