Here are a few websites:
You must be aware that the market is flooded with free books. In my experience, Amazon is the strongest. Through its KDP Select program you can offer a book for free for a certain period of time. In second place is Get Free Ebooks. From this site, my book Promise Me Eternity was downloaded by approximately 1500 readers in one month. On the website Free Online Novels the same book was downloaded only by around 250 readers.
From what I have managed to find out from various experiences on the internet, you can be very happy if your free book is downloaded by more than 500 readers. Everything over 2000 copies is very, very good.