It is not a free product, but very interresting stuff.
www.jms2win.com/en/download/jmsmultisites13
When you need to install and manage 20 times the same application, it will use only one file system, and simply manage the whole thing in database.
One upgrade for all.
One server for all. No need to manage shared host for the clients.
I don't have huge need at the moment because I dedicate my time on Cook, but since this summer, I need to host Cook applications, and doing so, I can resell the same product to other clients.
You do evolving your product as a service and all your clients benefits of the new features.
When you distribute a product application (on the JED), you need to create the whole environement (website, docs, support, ...)
When you rent your application, you simply sell a service and you are in charge of making it working for the client.
And you don't need to install again, and repeat the same.