Websites like KissManga and MangaFox are often used. They offer a large collection and are user-friendly. However, be aware of potential ads and the legality of some content.
AO3 (Archive of Our Own) is great. It has a large collection of well - organized anime fanfiction. FanFiction.net is also popular, with a wide variety of stories.
I wouldn't recommend looking for free manga on unauthorized sites. It's illegal and can harm the creators. You might try some legal platforms that offer free trials or limited free content, like Shonen Jump.