NewsBlur - http://www.newsblur.com - a visual RSS feed reader with intelligence. I have a few hundred paying users and a few thousands free users. I develop it almost entirely on the train, which I'm on for almost an hour and a half everyday.

It's a ton of work and is profitable in the sense that the hundreds of dollars a month in server costs are just a bit more than covered by the premium users. But otherwise, it did help land me a number of great connections, both in NYC and SF, where I just moved.

I code in the open. NewsBlur is entirely on GitHub: http://github.com/samuelclay. The iPhone app I'm working on is also there, so some folks use it as a way to send me issues, others go so far as to add their own pet features. It's kind of neat to see a community spring up around the code itself.