The TeX program
TeX software is available from a number of sources:
LaTeX is an integral part of all "standard" TeX distributions, and AMS-LaTeX is included among the "required" LaTeX packages.
Extended TeX engines
pdfTeX generates pdf output instead of dvi.
XeTeX supports the direct use of system fonts.
e-TeX provides extensions mostly of use to class and package developers.
LuaTeX is based on Unicode and embeds Lua (a scripting language) and MetaPost to permit operations not easily accomplished with the basic TeX engine; still under development.
Other TeX Resources
Other valuable resources for a TeX user
CTAN -- the Comprehensive TeX Archive Network, with locations around the world.
Use the CTAN search facility, implemented by Jim Hefferon, to look for the name of a file you need, or search Graham Williams' TeX Catalogue by keyword.
The TeX Catalogue contains concise descriptions of TeX and LaTeX tools and packages, mostly available from CTAN, and includes additional links to many informative Web pages and authors' home sites.
TLContrib -- an archive of contributed packages which, for various reasons, cannot be distributed in TeX Live (not free according to FSF guidelines, not available on CTAN, intermediate release for testing, ...)
Mercurial Repositories of CTAN -- an archive containing version control repositories with older versions of many CTAN packages (mainly LaTeX).
Why TeX?, a comparison of structured document input vs. WYSIWYG, by Allin Cottrell
Cross-platform TeX editors, a list compiled by the AMS
Frequently Asked Questions and other Q&A sites
These tools are included in TeX Live and other comprehensive distributions.
amsrefs supports the inclusion of structured bibliography data directly in LaTex source files
BibTeX resources via public-domain TeX sites