Use the HTTP+Swagger action with 3rd party services
Microsoft Flow extensibility is built on top of Swagger, an open standard for describing how APIs work. Because this is an open standard, any service exposing an API can integrate with Microsoft Flow with no more work* than creating a valid Swagger endpoint (* Note: authentication can potentially be an issue, only a certain set of authentication providers are supported today, please read more in our documentation).
A company called CData has exposed the swagger for their Cloud Drivers, which, as they say: "Expose live data from internal and external facing web applications, mobile apps, ETL tools, and popular desktop applications." If you puchase CData Cloud drivers you can now use their swagger to call them from a flow! In this blog post by Jerod Johnson, you can read about how to set this up. As you can see, the end-result is a native experience for any action that can be exposed through their Cloud Drivers:
We hope this helps to demonstrate how extensible Microsoft Flow is and how it can integrate with 3rd party services. If you know of other systems that support swagger, please share on the forum.