Amazon’s KDP Select program is one that helped propel several of my books (not just the ones that were free) onto Category lists and Popularity lists… i.e. it helped me sell a lot of books.
The idea is that you put your book for free for a couple of days, promote it like crazy to various websites, blogs, on Twitter and on Facebook. The free downloads propel the book up the Best Seller charts vastly increasing visibility and resulting in sales afterward.
I have an ebook available if you are interested in this type of promotion. It will walk you through step-by-step exactly what I did that helped my ebook go to #1 on the Free list and then hit #86 on the Paid list.