Baen has a long tradition of giving away free books to sell other books in the series. Now that they are on Amazon, they continue that tradition.
This search should get you all the Baen books that are currently free. For some reason sometimes you get a ringer in the search.
There is quite an assortment of free books from many of the Baen stable of authors. Science fiction, military fiction, fantasy.
Many of the books are the first books in a series. Reading them may well convince you to buy other books in the series, which is of course why they are giving these books away.