arXiv (pronounced "archive") is a repository of electronic preprints, known as e-prints, of scientific papers in the fields of mathematics, physics, astronomy, computer science, quantitative biology, statistics, and quantitative finance, which can be accessed online. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository. Begun on August 14, 1991, arXiv.org passed the half-million article milestone on October 3, 2008, and hit a million by the end of 2014. By 2014 the submission rate had grown to more than 8,000 per month. 

The arXiv was made possible by the low-bandwidth TeX file format, which allowed scientific papers to be easily transmitted over the Internet and rendered client-side. Around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Paul Ginsparg recognized the need for central storage, and in August 1991 he created a central repository mailbox stored at the Los Alamos National Laboratory which could be accessed from any computer. Additional modes of access were soon added: FTP in 1991, Gopher in 1992, and the World Wide Web in 1993. The term e-print was quickly adopted to describe the articles.
When did they clear half-million articles?
October 3, 2008