SaaS or Software as a Service is a technique that works from a variety of different operating systems and platforms.

You could say that SaaS is OS-agnostic. You are able to develop SaaS applications from most current operating systems. (If you are still learning about SaaS, here is a definition.)

Microsoft supports its .NET development envirment through its Windows Azure platform. Many cloud-computing platforms support a variety of different operating systems (even Microsoft's Windows Azure platform supports non-Microsoft open-source development environments).

For a complete list of SaaS companies, consult the cloud computing SaaS directory.