Interesting. How strictly can you filter the respondents using Mechanical Turk? And is it only by demographics, or also by interests and other criteria?
Reverse Craigslist - Allow a user to enter an anonymous Craigslist email address (that they had previously replied to) and their city, and then return the post that contained the email.
The reason being: When looking for something on Craigslist, I often send similar emails to several potential sellers. But then, when they reply, I have no way of knowing which posting was theirs (and the link to it). (Of course, you could contain the link to the posting you're referring to in each email, but that's a hassle that I'm sure most forget to do.)
@AwesomeReminders: You might consider offering AwesomeTextMessageReminders at a reduced rate. Either custom text, given by your customers, or something like: "Hi John. Someone who knows you told me you were awesome. Have an awesome day."
Learn to surf. Or snowboard. You won't master either, of course. But standings up on your first wave or learning to link turns on the snow is exhilarating, and the first step towards getting better.