My first application, Aspen, was multi-tenant. State agencies could alter forms, add fields, change printed reports and that's why it succeeded. About half of my projects have been multi-tenant.
We used Adobe Flex and the Cairngorm framework for this call center app but it lacks multi-tenant behavior, so I added a factory source to load different workflows (Front Controller) and data models (Model Locator). We replaced the service layer with Siebel and Oracle services from another project.
This diagram was a presentation for a board of directors. We load different attributes and workflows for two divisions, FSE and FSR. It's the same call center application with different data for FSE or an extra step for FSR or a different screen but 90% of the application is common. This application was eventually used by 15 divisions in the company.