This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
You're starting at the wrong end. The main point of SaaS is in the second "S" - service. You need to start with a concept of what kind of service you want to deliver, and how it's going to get delivered. Then you can start thinking about how to implement it. Two common approaches are web apps and web services - for which any number of frameworks are available already.