arXiv is the original preprint server, launched in 1991 and now hosting over 2.5 million papers in physics, mathematics, computer science, statistics, and related fields. Researchers upload their manuscripts before or alongside journal submission, making new work available months before it appears in a formal publication. The site is operated by Cornell University and is free to read and submit.

It has become the primary way many researchers in these fields share and discover new work. Categories like machine learning and computer vision are particularly active, with hundreds of new papers posted daily. You can subscribe to email alerts for specific categories, browse by subject, or search by author and title. For anyone following fast-moving technical fields, it's an essential daily read.