Probably put in a prior list, but for newbees, if you put "free science fiction kindle books" into the search engine at Amazon, you can get quite a few free science fiction including Andre Norton and David Drake. A lot of the other authors I do not know, so can't tell you the quality although quite a few are rated 4 star. Just be careful that it does indeed have 0.0 for a price.
On the negative side, you'll have more books in your TBR that you don't have time to read.