Best of Breed

In my humble opinion these are the best Open Source projects. This is not an exhaustive list. There are many fine open source projects. But these are top-of-mind for me.
 

 Drupal

 The best Content management System

 Apache

The best web server

 Postgresql

The best Database

 Firefox

The best Internet Browser

 Debian Linux

The best OS distribution

 GCC

The best compiler toolset