In most cases, I'm firmly opposed to this. For example, I've known several people in psychology and social services careers where they essentially worked unpaid for years before qualifying for their licence. I think that's absurd because it forces you into creating value for others without receiving any of that value back.

However, I'd argue that unpaid contribution to open source software as a way to build reputation is not absurd, because the value you are creating is still available for you to use and benefit from. Additionally, you are free to work on anything you like - something you can't do at an organization which is compelling you to work for free.

i.e. an internship