Reddit is a social news website (just like digg) on which users can post links to content on the web. Today (June 18, 2008), became an open source project. All of the code and libraries written for Reddit became freely available. Here's a quick list of the open source products that reddit is built and runs upon:
Debian, lighttpd, HAProxy, PostgreSQL, Slony-I, various python libraries, Psychopg, pylons, Solr, Tomcat, Ganglia, Mercurial, Git, gettext (translation), daemontools, and memcached
Today we're excited to announce that we're open sourcing reddit.
All reddit code is licensed under the Common Public Attribution License, which is basically the Mozilla license with a handful of changes. Specifically, the CPAL stipulates that when running reddit's code publicly, any changes to the code must be made available publicly and the site must make clear that it is running reddit code.
View Reddit code online here.