Archive of Our Own (most commonly abbreviated as AO3) has been running since late 2008, gaining widespread popularity from 2011 onward. It almost exclusively hosts fanfiction.
The inception of the Archive started after the decline of LiveJournal as a fanfic hosting platform, especially the notorious Strikethrough '07 incident, where LiveJournal responded to threats from a group of trolls purporting to be To Catch a Predator-esque online vigilantes by deleting a large number of allegedly "objectionable", but in most cases entirely innocent, journals and communities without warning, which resulted in many fans losing the contents of their journals. AO3 as a project of the Organization for Transformative Works was a response to this, and the goal of the 'archive of our own' is to ensure a safe archive that would not purge fanworks due to pressure either from Moral Guardians or aggressive copyright enforcers.
To give an indication of how young it is compared to the existing behemoth FanFiction.Net (which started in 1998), AO3 reached one million stories in total on February 15, 2014. The Harry Potter fandom on FFN had roughly 674,000 on its own as of the same day.
The rise in popularity has been helped along by the rise of Tumblr as both sites share a similar fandom mentality and outlook, as well as by AO3 being the official hosting site or encouraged posting site for a few popular fanfic fests and events such as Yuletide. It also gained popularity when Fanfiction.net had a crackdown on Mature Content Fics in June 2013 (the last time that website did such a crackdown was back in 2002). AO3 is very popular because it allows users to view fics as one single document instead of the separated chapters and even download stories as HTML, EPUB, MOBI, and PDF files, making it great for people who want stories on the go.
It also has a much less restrictive Review system than Fanfiction.net—like LiveJournal or Reddit, it uses comments which may be edited and stacked on at your leisure, rather than having a single use for each chapter that can never be edited, replied to only privately. Other than that, there are also some comment moderation settings: authors may accept all comments by default, only allow them to be shown publicly after approval, or simply turn them off altogether.
Also notable is the "Kudos" system, which allows someone to say that they like the work without going to the trouble of a comment, but still better than another tick on the view count.
The impressive usability and lack of censorship on AO3 has led to a massive and devoted coterie of users. So devoted that, in 2019, the entire website was first nominated and then awarded the Hugo award for Best Related Work. The Hugo Award is considered by many to be the premier honor in science-fiction, so just being nominated was an impressive feat for a website built and staffed mostly by volunteers. The award was accepted by noted author Naomi Novik, a co-founder and past board member of the OTW, and herself a multiple Hugo nominee.
"Tropes of Our Own":
- Adjustable Censorship: If you don't want to see certain content, archive warnings, ratings, or pairings as a reader, you can set the search to filter whatever you don't want to see from the results. It's one of the most comprehensive filtering systems of any fan-fiction archive, making it easy to find whatever you're looking for even outside of triggering or sexual content, such as a specific pairing, characterization, or AU scenario.
- Alan Smithee: If authors want to disassociate themselves from a work but don't want to take it down, they can choose to orphan the work. All orphaned works are moved to "orphan_account". Users who still want to retain some degree of control and be able to reply to new comments are advised to either add the works to the 'anonymous' collectionnote , or transfer ownership to a throwaway account using the "co-author" property.
- Arson, Murder, and Jaywalking: There are multiple statements in AO3's Terms of Service reiterating that the website isn't responsible for any content that harms, disturbs, or triggers the user, often rattling off a list of intense adjectives describing said content that always ends with "grammatically incorrect" or "badly spelled".Unless it violates some other policy, we will not remove Content for offensiveness, no matter how awful, repugnant, or badly spelled we may personally find that Content to be.
- Bowdlerise: Averted. Unlike FanFiction.Net, the Archive has minimal content restrictions, allowing explicit sexual content.
- Content Warnings: The Archive has four primary content warning flags: "Graphic depictions of violence", "Rape/non-con", "Major character death", and "Underage". Authors can also choose to select "None of these warnings apply", indicating the story has no major content warnings, or "Choose not to use Archive warnings", indicating that they have chosen not to forewarn readers as to the content. Additional content warnings may also be provided in the tags.
- Fake Relationship: Has its own tag, with over 40,000 fics written as of November 2022.
- Fan Webcomics: The archive allows you to share fan art and comics on it, although the images themselves cannot be hosted on the site and need to be embedded via external links.
- Fan Translation: The archive allows you to publish translations of fanfiction and has a few settings available that makes it easier to give the original authors credit.
- "Freaky Friday" Flip: Has its own tag, with over 8,000 fics under it as of November 2022.
- Full-Name Basis: The canonical character tags use that character's full name, nicknames and codenames included. For example, an Overwatch story might be tagged with "Angela 'Mercy' Ziegler" or "Jack 'Soldier:76' Morrison", or a Gravity Falls story might include "Jesús 'Soos' Alzamirano Ramirez". However, authors are allowed to tag their works however they like, and more minimalist tags like just "Mercy" and "Soos" can be redirected to the full canonical tag if they aren't ambiguous.
- MST: Unlike FF.net, this is allowed on the site and even has a legitimate tag dedicated to it.
- Porn with Plot and Porn Without Plot: Both tags on the site.
- Real-Person Fic: Unlike FanFiction.Net, fics involving real people are allowed on the Archive.
- Script Fic: Unlike FanFiction.Net, fics that use script instead of prose format are allowed on the Archive.
- Soulmate AU Fic: 32,000+ of them, sorted into subcategories based on whatever Red String of Fate is prevalent.
- Start My Own: Essentially the reason for the site's existence, with a dash of deliberately learning from history. The site is entirely funded by donations (no ads means no dealing with advertiser content policies), retains legal counsel (just having a lawyer on speed dial is often enough to fend off spurious legal threats to legitimately transformative works), and has a broad, clearly defined leadership structure with conflict resolution procedures (to prevent a site leadership implosion from taking the whole thing down from the top). All of these are lessons learned the hard way from other fandom sites.