I wrote it in eight days with a budget of $60, and recouped that within three sales. It has been profitable since. I don't know if you would call it a lot of traction -- by Twitter accounting I have something like two hundred thousand users. Anyhow, 4 years and 3,500 odd paying customers later, it was paying my full-time salary, and I quit the day job. That was about 7 months ago. BCC (and consulting) gave me enough time, money, and smarts to get my next project ready.

http://www.appointmentreminder.org

That launches in 48 hours. Depending on one's optimism in counting promised orders prior to the cash physically being in one's hand, it is already profitable.