Packagist is the main Composer repository. It aggregates PHP packages from public sources like GitHub that can be installed with Composer.
WordPress Packagist mirrors the WordPress plugin and theme directories as a Composer repository so they can be installed with Composer.
Private Packagist is a commercial package hosting product offering professional support and web based management of private and public packages, and granular access permissions.
Some of Private Packagist's revenue is used to pay for Composer and Packagist.org development and hosting so using it is a good way to support the maintenance of these open source projects financially.
Satis is an open-source, static Composer repository generator. It is a bit like an ultra-lightweight, static file-based version of Packagist and can be used to host the metadata of your company's private packages, or your own.
Release Belt is a Composer repository, which serves to quickly integrate third party non–Composer releases into Composer workflow.