I have listed all the sites that you can use to promote your kindle book when you create a free promotion. Some of these sites allow you to promote paid kindle books as well but usually there is a fee involved.
I shall update this list as and when I find new sites or notice that old ones no longer exist for this purpose. If you are at a loss to know where to start I would recommended beginning with the website Author Marketing Club.
While not exclusively for free KDP promotions, I would like to mention a site that encourages authors to promote their book by writing a guest article. This is another effective method for promoting your books for free. You can find out more by visiting: https://www.nfreads.com/
When promoting on Twitter and Google Plus remember to make the best use of hastags because this will help people who are searching to find your promotion.
Here are some useful ones to use:
Google Plus Communities
Other Useful Websites
Remember that you should always check the rules for any posting group.