Ask HN: What should I build (for OSX)?

I'm looking to build my fledgling Cocoa skills and I want to build an OSX app. What should it be? Here are my ground rules:

1. It must be genuinely useful, not just an academic exercise.

2. It must be small enough in scope so that a competent OSX developer could build it in about one month full-time.

3. Bonus points if it's something I can sell.

So what should I build?