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?