To aid the archiving and retrieval of of TeX-related files, a TUG working group developed the Comprehensive TeX Archive Network (CTAN). Each CTAN site has identical material, and maintains authoritative versions of its material. These collections are extensive; in particular, almost everything mentioned in this FAQ is archived at the CTAN sites (see the lists of software at the end of each answer).
The CTAN sites are currently \FTP{dante.ctan.org} (Germany), \FTP{cam.ctan.org} (UK) and \FTP{tug.ctan.org} (USA). The organisation of TeX files on all CTAN sites is identical and starts at tex-archive/. Each CTAN node may also be accessed via the Web at URLs http://www.dante.de/tex-archive, http://www.tex.ac.uk/tex-archive and http://www.ctan.org/tex-archive respectively, but not all CTAN mirrors are Web-accessible. As a matter of course, to reduce network load, please use the CTAN site or mirror closest to you. A complete and current list of CTAN sites and known mirrors is available as file CTAN.sites on the archives themselves.
For details of how to find files at CTAN sites, see "finding (La)TeX files".
The TeX user who has no access to any sort of network may buy a copy of the archive as part of the TeX Live distribution.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=archives