Not exactly what you are looking for, but Basecamp was a side project of 37signals. They worked on it about 10 hours a week, because they were doing consulting/client work for the majority of their hours and pay. As Basecamp made money they gradually moved off of client work. Read their blog and Getting Real for more details, but it's full of useful stuff around this. Basically, they needed Basecamp for themselves, so had a lot of motivation to get it done.
In my own personal life, I've had mixed success. When I was working full time I tried to start a little piece of database management software for Java PDAs. I never finished it. It was hard to stay focused and just get the thing done. One of the main problems here was I was working on something I didn't have an urgent need for myself.
I learned a lot from that, and the next "side project" I did was just consulting work for someone else. Moonlighting in other words. That went a lot better. Probably because someone else wanted my work. So I had to get it done. That was easy for me to bang out things people could use right away.
Tgethr is a side project of ours now at Inkling. So we basically have full time jobs doing what we have done for over 4 years now, but spend a little time building this other application that helps us out with the majority of our work. It's definitely a challenge to work on side projects. There's so much I want to improve on it, but we don't have the time when our main work beckons for our attention. But gradually things get done, and tgethr is starting to kick some ass. We used some holiday downtime to release some improvements to our site there, and it's paid off greatly with us increasing our paid customers by 75% even over the Xmas + New Years weeks.
The main point of advice that comes from this is: I hope your side project is something you feel you yourself urgently needs. If it's not, it's going to be super hard to stay on it. So many things will distract you and it will be easy to drop your goals. But if this project is something that's going to help you do something better right now (work better at your current job, manage a hobby or club better, etc.) you'll stick to your goals so much better.
Ask yourself this question: If this data-mining project was built by someone else yesterday instead of you, would you drop your money on it today because you need it that bad? With tgethr, I was paying for someone else's tool because I needed it that bad. But that other tool didn't cut it, and we needed to start our own.