Calibre is the leading eBook and eReader management tool, and that's not the usual marketing nonsense. It supports conversion from and to a wide variety of eBook formats. It will also assist you in organizing your eBook library and transferring your eBooks to your eReader (almost all are supported).
Beta support for the new format was started last week, with official support available today. You can make a KF8 file from any of the ebook formats supported by calibre, including the KF8 ebook created by Amazon's own KindleGen. That particular file contains both the KF8 as well as the older Mobi file, and is often rather bloated by the excess content.
With today's update you will also be able to tweak the KF8 output to fine-tune the result of a conversion. Note, though, that this option only produces the KF8 file, and that isn't supported on all of the Kindle device or apps.
But it's still a better option than using KindleGen to produce a KF8 ebook; that will give you a bloated file.