Selling support or related services is one way, I think Stallman gave the example back in the day of how he made money through selling physical copies of software (before online distribution was universally viable). The software was free and could be re-distributed, but a profit could be made from providing the service of doing the distribution.

On a bigger scale (although they're not so popular at the moment), historically Red Hat has been the go-to example for how to make money in the spirit of free software. They fund and contribute to many upstream FOSS projects, and in return they can make a fortune out of selling commercial support for that software, while the software itself is still free.