I am currently running 4 blogs, hosted on the same server with different installations of WordPress. On the other side, I have started running multiple site on a single installation of Silvesrtripe, my favourite Content Management System. The conclusion is evident : running multiple sites on a single installation is a technical challenge but saves a lot of hours on regular tasks of securing, maintaining and developping on a specific platform. Of course, this kind of exercise comes with its own risks : you can get all your sites down in a single error or successful hack attack. But on the other hand, restoring backups would be a single click affair. With WordPress, the MU version enables the devlopment of multiple sites, and we have a full case study, published by WordPressPublisher, inspired by work from Tierra Innovation and WNET.ORG (Channel Thirteen in New York) , who achieved multiple site development WordPress MU as a CMS for WNET.ORG’s. The two companies also publish a white paper : How a Non-Profit Media Company Profits from Building Open Source Online Publishing Platform (PDF). Magento, the new and revolutionnary opensource ecommerce soft, also allows some level of multiple site building : again, we have a technical challenge here but the few days learning curve is worth the effort.