Some years ago, a TUG Technical Council working group defined a common network archive for TeX software, the Comprehensive TeX Archive Network (CTAN). It has become the central repository for TeX-related software on the Internet, and is the source for distributions such as TeX Live and MiKTeX.
CTAN was initiated and continues to be supported by the TeX user groups worldwide. Please consider supporting them in turn, by joining TUG (or the TeX user group best for you) or making a donation, which you can specifically target to CTAN projects.
Here are some ways to access CTAN, directly or indirectly:
To receive email about updates to CTAN holdings, subscribe to the ctan-ann mailing list. There's also an RSS feed for CTAN. Archives of ctan-ann are available at Gmane and mail-archive.com.
CTAN welcomes contributions of all sorts. For more information and links on uploads, please see the FAQ entry on contributing a file to the archives.
A snapshot of the archive is distributed as part of the TeX Collection (on DVD) approximately yearly by TUG (as a benefit of membership), and many other TeX user groups. You can also purchase the DVD without becoming a member.
The backbone servers at cam.ctan.org (UK) and dante.ctan.org (Germany) allow uploads. CTAN is mirrored worldwide. Access via http, ftp, and rsync are all supported in various combinations at the various sites; the backbones support all three.
To make most effective use of the mirrors, please consider using the hostname http://mirror.ctan.org to identify packages on CTAN; for example, http://mirror.ctan.org/systems/texlive/. This automatically redirects to some relatively nearby CTAN site.
(This feature was created by the late Randy Kobes at the University of Winnipeg. We thank him profusely, in memoriam. He also set up a similar automated redirection for GNU mirrors, and searching for CPAN packages.)