Wikipedia launches
An encyclopedia anyone could write opened, and within a few years it was the largest organised body of text in open access.
Why it matters
What language models would train on was created by accident: text that is large, structured, multilingual and freely licensed.
Larry Sanger proposed a wiki beside Nupedia on its mailing list on 10 January 2001: a site where "absolutely anyone" could make changes. The name Wikipedia appears the next day, and on 17 January he announced wikipedia.com on the list. The contents were released under the GNU Free Documentation License. The intent was purely editorial and had nothing to do with machine learning. But the combination of size, cross-references, categories and a free licence made Wikipedia the standard source for everything from training word vectors to fact-checking sets. Practically every large language model has it in its training data.