4chan
An anonymous imageboard started in 2003 by fifteen-year-old Christopher Poole ("moot"), built as an English-language copy of Japan's Futaba Channel. No accounts, no names — everyone posts as "Anonymous".
The site consists of topic boards and threads that are pruned once they fall off the last page, so there is no archive and no history attached to an identity. The combination of anonymity and impermanence produced a culture of its own where phrasings and image formats mutated fast, and a disproportionate share of internet memes from the 2000s and 2010s originated there. The same properties have made the /b/ board and later political boards a launching point for coordinated harassment campaigns, and the site has repeatedly been linked to radicalization and to attackers who posted manifestos there. The Anonymous collective emerged from the same environment.