The source code for the W3C Markup Validation Service is available under the terms of the W3C Software License.
If you just want to glance at the code, or see its revision history, you can browse it directly in Github.
The most interesting files are currently a CGI script called "check" that does pretty much everything, and possibly also the httpd.conf configuration file snippet for Apache. Select the topmost revision numbers on these pages to see the most recent revision of each file.
Fedora RPM packages of the validator are included in Fedora. The name of the validator package is w3c-markup-validator, use the standard automated package management tools of the distribution (such as yum) to install it along with its dependencies.
For Red Hat Enterprise Linux and derivative distributions, the w3c-markup-validator package is available in EPEL.
openSUSE/SUSE Linux RPM packages of the validator are available, courtesy of Sierk Bornemann, at software.openSUSE.org, <http://software.opensuse.org/>. Starting with openSUSE 10.3, the latest stable validator package and all its dependencies are included in the official stable openSUSE distribution. The name of the validator package is w3c-markup-validator, use the standard automated package management tools of the distribution (such as YaST, zypper, smart, apt4rpm or yum) to install it along with its dependencies.
Additionally, you can also get these and other needed packages from the openSUSE Software Repository at <http://software.opensuse.org/package/w3c-markup-validator>
A Debian package is available, courtesy of Frédéric Schütz.
Starting with Debian 3.1 ("Sarge"), the package and all its dependencies are included in the official Debian distribution, and can be installed by running the command apt-get install w3c-markup-validator as root.
The Validator is also packaged as a standalone Mac OS X Application, called Validator S.A.C., courtesy of Chuck Houpt.
For instructions on how to retrieve the code using Github, see our web-based interface to the repository.
The purpose of making the source code available is partly to allow others to set up mirrors of the service (either publicly or within an intranet behind a firewall), but also to allow us to collaborate on making the service better! If you are interested in helping with the development, we suggest you first install the code and read our instructions for developers.