Apache Web Server, GNU/Linux Operating System, MySQL Database, Mozilla's Firefox Browser.
All pillars among the open-source community.
Each of these deserves its imminent position as a venerated project. Each has changed the world, and not a little. Moreover, they are the projects that spring to mind when we seek to justify the brilliance of the open-source licensing and development models.
But if this is intended to be a list of the highest-impact and most significant open-source projects, there is a project missing from this list.