AO3 rocks, as does the parent organization, the Organization for Transformative Works. They do a lot of pro bono legal work around free speech and copyright (which can be especially sticky around transformative works). AO3 has drawn ire and appreciation alike for their fierce free speech stance (which is, essentially, if it's against the law to exhibit in the state of New York (or against their TOS e.g. not a fanwork), it must come down. Everything -- EVERYTHING -- else stays.)

I also love another of their programs, Fanlore[0], a treasure trove of obscure internet fanfiction works, social groupings, and tropes. Sort of a combination oral history + TVTropes-esque fanfic breakdown.

Fanfiction is a beautiful thing and a great way to stretch creative writing muscles that can be helpful by being a bit less of a blank canvas than needing to invent characters and locations. Of course, a TON of fanfiction does invent, liberally, but I've seen people insist they can't write creatively, then make something beautiful when you ask them to rewrite a scene in their favorite show or book that they were disappointed with and boom, they just wrote their first fix-it fic[1].

Also, praise be for AO3 having no news feed/algorithm, and instead an incredibly intricate tagging system. Makes it easy to find what you want, avoid what you don't (see free speech commitment), and feel liberated from algorithmic exhaustion.

[0]: https://fanlore.org/wiki/Main_Page

[1]: https://fanlore.org/wiki/Fix-it