For some time now I've been promoting a few books by making the ebook version free. It's a long game, hoping that some of those who download and read the free books might be interested in the moderately priced other books in the canon.
The promotion has worked moderately well - it generates more sales of paid for books than I might otherwise experience. How long it will run for depends on a number of factors, but right now you can legitimately 'buy' some of my books for nothing from Barnes and Noble, Apple, Kobo, Smashwords and probably a number of smaller outfits. Just mosey along to your favourite ebook store and have a look, 'buy' a free copy or two if they spin your wheel, don't forget to leave a review sometime in the future and consider purchasing one or more of the others.
All of the above is true as long as your favourite ebook seller isn't Amazon, though. I don't have a problem with Amazon - I use them for a lot of purchases including ebooks - but because I decline to sell exclusively with them they decline to let me promote my ebooks in any meaningful way.
However there is a way around this, and it'd down to Amazon not allowing themselves to be undersold. If a customer finds a book for sale for less than Amazon, then they'll price match. I noticed one of my books popping up on the sales dashboard the other day as a free download, so someone has noticed that one of Amazon's competitors are selling it for free. It's in the US and I've checked the US website and they're listing it for $3.91, so it looks like you're going to have to ask. At least one other person has noticed this too, as I've just had another free download listed today, for the same book. Might be word of mouth, might be coincidence.
I'll leave you to find out which books are eligible, don't want to take all your fun away, but until I lift the promotion please feel free to badger Amazon to price match.
