While not technically free, since you pay for Amazon Prime, Amazon Prime Reading comes with that membership. This is the most recent place for free books I have used. I have found several books here, proving that free doesn't always mean shit writing.
Don't sign up at BookBub unless you want to enlarge your To Read list by several books per day. I'm not even kidding. I have to unsubscribe every once in a blue moon because it kinda breaks my heart that I can't read everything that lands in my inbox.
Admittedly, I haven't used Kobo as much as the other two but come on, I'm a girl with a 7K book long To Read list. I'll get to it! In the meantime, go for it. How could you regret free books?