Start an Apache Web Server in Mac OS X Mountain Lion

In OS X Mountain Lion the Web Sharing preference panel has been removed. The Apache web server remains bundled with Mac OS X though, but you’ll need to turn to the command line to enable the web server.

1. Create and edit the following file /etc/apache2/users/USERNAME.conf (replace "USERNAME" with the actual account)
2. Its contents should be as follows:
   <Directory "/Users/USERNAME/Sites/">
Options Indexes Multiviews
AllowOverride AuthConfig Limit
Order allow,deny
Allow from all
</Directory>
3. start the actual server: sudo apachectl start
4. accessing http://127.0.0.1/ will simply display It works!
5. files stored under /Users/USERNAME/Sites/ will be accessible from http://127.0.0.1/~USERNAME
6. stop/restart your Apache web server with sudo apachectl stop|restart

Source: http://osxdaily.com/2012/09/02/start-apache-web-server-mac-os-x/

More info here: http://reviews.cnet.com/8301-13727_7-57481978-263/how-to-enable-web-sharing-in-os-x-mountain-lion/
and here: http://coolestguyplanettech.com/downtown/install-and-configure-apache-mysql-php-and-phpmyadmin-osx-108-mountain-lion