Interactive transformation tool

You tagged this question with fmeobjects, so I'm going to go with that.

For a Desktop only implementation I think you would create a workspace in FME Desktop that carries out the transformations you need. Then in your interactive tool (create it using Python, Java, C++, .NET) you would call the workspace and pass data to it using the FME Objects IFMEWorkspaceRunner method.

For a Server implementation you would create the same workspace, the same way, but then publish it to FME Server. Then you would create an interactive web tool (Javascript, Python, etc) and in there call the workspace using a method in the FME Server API [something like FMEServer.submitJob( repository, workspace, parameters, callback )] instead of FME Objects.

Hope that helps. There's a very wide range of possibilities that could meet the needs of your rather broad question, more than just my suggestions.

I think the suggestion from Mark is the way to go. It is inevitable that it'll be (much) more complex to implement something like it on FME Server than Desktop, as the Server isn't really targeted for user interaction during translation.