One of the traditional issues with software-as-a-service products is limited ability to customize. That's one of the issues Salesforce.com is looking to tackle at this week's Dreamforce Conference when their new Force.com, or "platform as a service", is combined with Visualforce, a set of tools for "user interface as a service". As you can see in the illustration below, Salesforce is looking to create a complete top-to-bottom stack of hosted services from low-level infrastructure to high-level UI design and development.
With VisualForce developers and ISVs will be able to use HTML, Ajax, Flex and Salesforce's Apex to create hosted custom applications that don't look at all like traditional Salesforce.com applications, like in this example of a custom insurance application:
The developer preview starts this week with general availability in the fourth quarter.