It's called Mobi Unpack, and it's been around for a couple years now. It originally just took apart a Kindle ebook and gave you the source files, but the last few version have also had the ability to take apart a KF8 file.
In fact, the developers of this tool have been working on taking part KF8 files ever since I posted the first demos; they just forgot to tell me about it. They have it working so well that it will hand you a KF8 file (only works on the Kindle Fire right now), a Mobi7 file (works on all Kindles and apps) and the source content.
Mobi Unpack and the instructions you need to run it can be found here. The tool is a Python script, so you'll need to download and install a python app to run the script.
MU can do several things, but if all you want is to split up the KF8, Mobi7, and source then click the "split" box. Assuming it doesn't choke on the file you give it, it should work quite well. I've tried it with a couple files, and they opened just fine afterwards.
Now that you can split the files up, I'd suggest that you throw away the older Mobi7 that came out and that you keep just the KF8. The Mobi7 file will look like a poor automated conversion from Epub (assuming the source Epub was high quality). It's better to continue to make a Kindle file the old way. You'll get a better product.