GitHub
GitHub is a hosted source control provider that delivers hosted Git repositories. GitHub allows anyone to create as many publicly visible repositories as they want. Only, when you create private repositories that require three or more contributors, you must switch to a paid subscription.
This model that allows unlimited, free usage of the platform if developing in public has made GitHub, by far, the largest host of open source software in the world.
GitHub was acquired by Microsoft in 2018 and since then, Microsoft and GitHub have worked together to create a great integration experience between GitHub repositories and Azure DevOps, specifically with Azure Boards and Azure Pipelines. In addition to this, Microsoft has stated that GitHub and Azure Repos will continue to exist next to each other and that there are currently no plans to terminate one of the products in favor of the other.
There is also an on-premises offering by GitHub called GitHub Enterprise.