This is a .app wrapper around KindleGen using Platypus. You can drag and drop an OPF or an EPUB onto it and it will show you the build output in a nice little popup. Shooting for simplicity.
Alternatively, you can just use Kindle Previewer [http://amazon.com/kindlepublishing] to convert your content to Kindle Format 8, as it has KindleGen built in.
KindleGen is owned and licensed only by Amazon. Please visit [http://amazon.com/kindlepublishing] to download a copy of the program.