See What's Really Happening in Physics -- arXiv.org
Started for the High Energy Physics community in 1991 at Los Alamos National Lab under the name xxx.lanl.gov, and with a purely e-mail interface(!), arXiv moved to Cornell Univerisity with its creator in 2001.
The archive includes preprints from all areas of physics research, not just HEP, as well as computer science, mathematics, and quantitative biology. "Preprints" are a way for the research community to get their results out for everyone to read, during the often lengthy period (months, sometimes!) between submission to a journal and actual publication. In many fields, it has been traditional for decades that printed copies of "preprints" were mailed out, sometimes in regular batches, between major University departments. That mechanism has been superseded almost exclusively by the arXiv database.
Besides these traditional pre-publication journal articles, the archive also often includes conference proceedings, and submission of much older published papers of general interest to the community. There's also (usually in the general "physics" category) a scattering of