So the answer depends. If you are working in Enterprise America, then you have to start contributing to open source projects on your own and outside of your "day" job.

If you are working at a small company or startup that is using other open source projects/products, you can contribute to those projects to help improve your company and get paid to do it.

In the end, if you are hacking on something you think is neat or interesting, put it out there. The worse thing is no one notices, the best is you now have things you can point to.

Personally, I use a combination of all the above, yet I am the first to admit, I screwed up early on in that I used a company email/GitHub account I had to do most of my contributions instead of my personal email. Had I used my personal it would have made my contributions easier to track, but either way they are provable.

Either way, the more you put yourself out there the better. Good luck!

While working on a commercial project, you typically have to write some code that isn't related directly to the core business. This code can be open sourced as a library.

You should not feel discouraged. If you consider the whole "white board exercise" you might think of open source github projects like that. Bits of code you can write to learn about bloom filters, or try comparing sorts. Also you can 'fork' an open source project and submit pull requests from it on Github.

But it does pre-suppose that you like to dabble in writing code in your "free" time.

I don't worry about potential employers. I've done a bit of open source for fun. But now I can't be bothered working for free and would rather do commercial stuff for side projects. Happy to show a PE some of the code, but it ain't going to be on Github.