Drupal
Creating a drupal multisite install on a2hosting.com
It took a little while but here is the formula for creating a multisite install of drupal on a2hosting with shared hosting: 1 install base with each site having its own database.
I am assuming you maintain all the addon and subdomains and have full control over them. Thus all extra non-core modules can simply be put under the main drupal install so it doesn't matter if they are enabled or not in each drupal site.
1. Install drupal in the main public_html directory and ENSURE it is working first:
/public_html/files
/public_html/includes
/public_html/misc
/public_html/modules
/public_html/profiles
/public_html/scripts
/public_html/sites
/public_html/themes
/public_html/.htaccess
/public_html/CHANGELOG.txt
/public_html/INSTALL.mysql.txt
/public_html/INSTALL.pgsql.txt
/public_html/INSTALL.txt
/public_html/LICENSE.txt
/public_html/MAINTAINERS.txt
/public_html/UPGRADE.txt
/public_html/WS_FTP.LOG
/public_html/cron.php
/public_html/index.php
/public_html/install.php
/public_html/robots.txt
/public_html/update.php
/public_html/xmlrpc.php
2. Create your subdirectories - one for each site. Examples:
/public_html/addondomain
/public_html/subdomain