I have been working as a contractor/consultant/hired gun for a decade now, and before that as a programmer since 2004. I do not recall ever signing a contract

I have been working as a contractor/consultant/hired gun for a decade now, and before that as a programmer since 2004. I do not recall ever signing a contract that said I couldn't do open source work or side projects. Now, there is often something that says I can't work for, or as, a competitor in the particular market they're in. Which, to be honest, is only fair. But I would not sign an employment contract that said I couldn't do side projects or open source work, generally.