It's no secret that MySQL is widely deployed in most leading web sites. Everyone knows how successful the LAMP stack has become and that companies like Google, Yahoo, Sabre, Evite, Citysearch all use tons of open source software (including MySQL) to scale their operations. As important as all of these companies are, I'm most proud of the fact that MySQL is used at Wikipedia. This is one of the top web sites in the world and not only is it built on open source software (Linux, Apache, MySQL, Lucene) and the MediaWiki project.
Wikipedia is famous not just for being a highly scalable open source project, but more importantly, it is an open and collaborative repository of human knowledge. There are more than 1 million articles in the english language edition of Wikipedia and there are smaller versions in more than 100 languages. And all of this is done with a staff of 5 employees (2 FTE). The bulk of the work is done by volunteers. The management of the systems, the software, the articles, the editing, you name it. Whatever criticisms there may be, Wikipedia has proven the value of an open, collaborative model and I'm proud that MySQL is a part of it. The Fortune article below discusses the distributed organizations of both MySQL and Wikipedia.