1. ereaderIQ: This place is pretty awesome. This is a recent find of mine but I take full advantage of this. You can sign up for a daily email or just go on the website. Here you can track authors so you know when a book goes on sale. They have tons of books and once you've gone through this list you just might end up seeing repeats at other places since this place has the most at least that I've come across. I think Digital Book Spot might be second in the running. You can go through the freebies or find out what books are now on Kindle and they also have books that have been marked down but will cost you money.
2. Digital Book Spot: You get a daily email with a link to a list of books. There are quite a few of them but I've come across errors so make sure the book is free before you buy it. There were a couple times when the book wasn't free but a few days later I was able to get both of them for free which is better than paying $4.99 for each one. I've saved quite a bit of money and even got a complete series of books for free with over two thousand pages and the regular price was $9.99. This is a combo of books that are on sale for free or for a lower price.
There are more but if you don't want to have a ton of emails from places I would suggest going with these two since the others will lead to more repeats. I put up with that since I have come across either a good deal or a freebie that I didn't come across in the first two places.
3. Read Cheaply: There aren't as many but this is another that is a listing of books that are either on sale for free or for a lower price.
4. My Book Cave: I really like this place because you can get an idea of what's in the book before you click on it. It has setting so you can decide what you want to have available to you. I can only speak for myself but it can be annoying when I think a book is going to be about one thing only to have it turn out to be something else like that book that was supposed to be witchy but for the first six chapters at least it was like reading porn which isn't my thing.
5. Book Bub: This is a good one. How often can you get a Brandon Sanderson book for $1.99?