As of January 2012, Debian is currently the most popular Linux distribution for web servers. CentOS is a close second and Ubuntu is gaining popularity.
Debian 29.7%
CentOS 29.1%
Ubuntu 18.1%
Red Hat 12.4%
Fedora 5.1%
SuSE 3.0%
Gentoo 1.2%
Source: http://w3techs.com/blog/entry/debian_is_now_the_most_popular_linux_distribution_on_web_servers
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