If you look around, projects that address some kind of a need tend to do quite well for themselves. That’s a good place to begin – by pinpointing a problem that you or someone else has and basing your project on one or more solutions for it. For example, my post about turning your browser into a central workspace was born from my efforts to fix my messy workflow.
The problem you’re attempting to solve can be a major one involving, say, the physical stress caused by working at a desk or a relatively minor one involving a malfunctioning iPhone button. What matters is that you offer a practical solution that is fairly easy to replicate. The best way to find out what others are struggling with is by keeping tabs on forums and social networking profiles that are closely linked to the kind of work you do.