KDP Select. (See link for a blog post I wrote a while back explaining how it works). That means you get zero, zilch, nada for each and every "sale".
Why would I want to give my work away for free? Authors say making the first book in a series free generates sales of subsequent books. That may indeed be true for them and if so, I'm happy for them. But I just can't get my head around the concept. I don't want to just sell book 2 (and book 3 soon) of my crime thriller series - I want to sell all 3 books. If I want to make one free, I'll do it when I want and for as long as I want - via Smashwords, which will let me set my own prices.
My books aren't exactly expensive at full price. I work hard to write them - hundreds, if not thousands of hours. I want people to make a decision to buy and hopefully read my books, not download a freebie and probably never look at it. My ebooks cost less than the price of a cup of coffee. Anybody who doesn't consider that one of my books is worth the price of a cup of coffee is not my target audience.