I don't know if this is old news or not, but I never see anyone talking about it.
My new favorite place for cheap and free ebooks is bookbub.com. You give them your email, select which kind of ereader(s) you have and then mark off which genres you are interested in. They send you one email a day with cheap and free books that fit your categories. There's always at least one free book in my emails and nothing has been over $3.99. I've also never received any spam or ads from them.