superstar projects, and associated implementation languages

C

  • GCC
  • GNOME
  • PHP
  • Linux
  • CPython

C++

  • Blender
  • Firefox
  • KDE
  • OpenOffice.org/Libreoffice

C#

  • Stack Overflow (non-FLOSS)

Java

  • Eclipse

Python

  • Youtube (non-FLOSS)
  • Disqus (non-FLOSS)
  • Django
  • Mercurial

PHP

  • Facebook (non-FLOSS)
  • Wikipedia
  • Wordpress

Ruby

  • Ruby on Rails