Bookstrand is limited author’s but has over 400 free books!!!! I even found Selena Kitt and Lynn Hagen
Ellora’s Cave is a great site for all manor of erotica. Also a great place pick up some free reads
If romantic erotica is more your speed then Totally Bound has it. A wide range of Authors and even some free reads. They also have the option to download to your Kindle.
Amazon has lots of free books and great prices on all your favorite Kindle products.
Itunes always has a great free list that updates regularly