Magento is one of the biggest open source ecommerce shops. It has a modular architecture. That means extensions can be provided as Magento modules. Some of the core modules are available as composer packages. The Magento Composer page is hosted here http://packages.magento.com/ via Satis.
Now VersionEye is crawling http://packages.magento.com/ once a day. Some of the packages are listed here. Now you can follow these Magento packages and you will receive automatically a email notification as soon a new version is available.
Beside the Magento repository VersionEye is crawling the Firegento repository as well. All together VersionEye is tracking more than 50K PHP packages on a daily basis.