I write mostly .NET code and store all my config information in my app.config file for development and in my web app application settings in Azure for test, QA, and prod.

Semi-related to your question:

I'm actually working on a service that provides a repository and collaboration area for i18n config files and I was just thinking about adding a more general config files repository feature. Would you be interested in a service like this that managed settings files for different environments in one repository?

I would - but just to be clear - the setting files should not be stored in the code repository.

With a mix of sensitive information like API keys, Vault might be a good solution...there's nothing preventing storing non-sensitive information like configuration in it. On the other hand, the business logic of API keys is different from config.

https://www.vaultproject.io/

Zookeeper?