To quit my job.
Since this is tagged with motivation, I don't think that money (on their own), is quite enough to carry yourself through building a new (good) product. The projects that I'm really interested in are the ones that I do for myself -- something that my friends and I will use regularly and enjoy. I figure that if the result is good enough, some money will come as a side-effect.
Solving a big problem better than anyone else has been able to do so before me.
To make the lives of a whole lot of people a whole lot easier.
Solving a problem for the teams I work in - even if I never get a customer, I'll at least have one user! (ie. me!)