I am responsible for delivering portal solutions to customers.  Today I do so by generating HTML pages (with JavaScript, CSS, etc..) that use AJAX to make API calls to our core platform.

Each customer portal must be a distinct with there being no "bleed over" between portals, no searching for other portals, etc.., is possible.  

I am considering open source CMS solutions that can simplify the process we are following and:

• Supports the "isolation" we need to have.
• Provide pre-built 3rd party components and add-ons that will enable us to provide more functionality without the need for us to build it.  Clearly the developer community here meets this need.
• Has a solid support structure in place (either via an open source community or otherwise).