arXiv.org is an archive for electronic preprints of scientific papers in the fields of physics and mathematics, originally hosted at the Los Alamos National Laboratory, now hosted and operated by Cornell University and mirrored worldwide. The archive was originally developed by Paul Ginsparg.
arXiv.org was formerly known as xxx.lanl.gov but it changed its name when it was found that censorware programs were blocking access to it from various sites, under the impression that the three letters 'X' in its name implied that it was a pornographic site. The existence of arxiv.org was one of the precipitating factors that led to the current revolution in scientific publishing, with the possibility of the eventual disappearance of traditional scientific journals.
A popular way to access the mathematics portion of arXiv is through the University of California, Davis's Front, a web portal that offers powerful search functions and a more user friendly interface. For this reason, the mathematics arXiv is referred to, by some mathematicians, as (the) Front.
See also: Public Library of Science