arXiv.org - The arXiv (pronounced "archive") is a repository of electronic preprints of scientific papers in the fields of mathematics, physics, astronomy, computer science, quantitative biology, statistics, and quantitative finance, which can be accessed online.
Einstein Papers Project at Caltech - The Collected Papers of Albert Einstein presents the first complete picture of a massive written legacy that ranges from Einstein's first work on the special and general theories of relativity and the origins of quantum theory, to his active involvement with international collaboration and cooperation, human rights, education, and disarmament.