I think you'll find a pretty large majority of HNers do that, either just for fun, or in the hope of eventually building a business out of their little projects

I think you'll find a pretty large majority of HNers do that, either just for fun, or in the hope of eventually building a business out of their little projects. I work for a large game developer by day, and by night I go home and write my own open-source game engine, as it lets me learn what I need to learn. It's very hard to experiment with new techniques at work, but it's important to my skill development. Also the codebase at work is so horrible, that working with a nice, concise C codebase in the evenings is positively pleasant by comparison.