This is a short list (not difinitive) just to be used as an example set.
- OpenStack
- Linux kernel
- Drupal (including thousands of contributed modules)
- Wordpress
- Ubuntu
- PHP
- Python
- Go (the programming language)
- MariaDB (very popular fork of MySQL)
- ImageMagick
- FreeBSD
- Chrimium (the open source project behind Google Chrome)
- Firefox