I am very excited by the recently-published KML Interactive Sampler (read the announcement). Using the Google Earth plugin, it allows you to test sample KML and see this content immediately in Google Earth. You can also play with your own KML. For example, you can copy and paste KML into the main field, click Update Earth and instantly see how your code will appear to end users.
For now, you'll need Windows to use this tool, as the Google Earth plugin is currently only supported on this platform. Rest assured that a soon-to-be-released Mac version is in the works.
Learn more about using KML.