Holiday Deals is another example of a site that I built using a single code base that can be used for an unlimited number of sites. Each site can have their own design with their own css and javascript files.
For the front-end I'm using the Savant OOP template system. For the back-end I'm using the Zend Framework where each client has their own login to the admin area to edit their own site content.
On the server I modified the Virtual Host file for each site that was needed and added an alias that would enable the code base to work for multiple sites.