Is there an existing license that is copy-left by default, but allows use in proprietary software through case-by-case negotiation?

Question

I'm looking for a license which is designed such that anyone can use my code so long as the license they release it under is copy-left. However, I'd also like to be able to license my code out for the production of proprietary code if the opportunity should arise. The idea being that I don't completely shut out the prospect of my code being used in proprietary software, but feel I should benefit if/when it is used in proprietary software.

Answer 1

This is known as dual-licencing, and it is a very normal thing to do. Since you are the copyright holder, you are not bound by the license, and may offer your code to any third party you choose on any terms you choose, including proprietary ones.

Note that once you accept other people's contributions to your codebase you are no longer the sole copyright holder, and you can no longer dual-licence without the use of a contributors' licence agreement. See eg How to accept contributions for AGPL / proprietary dual-licensed projects? for more advice on that, including on the possible chilling effects to potential contributors.