Nothing beats the thrill of stumbling upon a steamy read without spending a dime! My go-to for free smutty books is definitely Archive of Our Own (AO3). The tagging system is a godsend—you can filter by pairing, tropes, or even kinks, which makes it super easy to find exactly what you're craving. The quality varies since it's user-generated, but some fics are so well-written they put published novels to shame. I once spent an entire weekend binge-reading a slow-burn enemies-to-lovers fic that had me screaming into my pillow.
Another gem is Smashwords, especially for indie authors experimenting with shorter, racier works. They often run promotions where entire series are free, and the 'Adult' filter helps narrow things down. Just be prepared to sift through some amateurish covers—don't judge a book by its pixelated abs! For classics with a side of spice, Project Gutenberg has pre-1923 erotic lit like 'The Decameron,' though it's more poetic euphemisms than modern-day smut.