[Last updated 02 Jan 2014]
There is a Usenet newsgroup devoted to TeX and LaTeX:
There is a pretty good site devoted to questions and answers related to (La)TeX: The TeX-LaTeX Stack Exchange.
A FAQ about TeX is available, with searchable index. ("FAQ" means "Frequently Asked Questions")
CTAN is the Comprehensive TeX Archive Network. You can find almost any available macro file, support program, or free TeX system for almost any platform there. There are currently three hosts, one in the UK, one in Germany, and one in the USA. The link above connects to a "mirror selector" that tries to send you to a nearby mirror.
This site can also be accessed through the TeX User Group web page. Look for a link to CTAN.
My involvement with TeX extends to associated programs, including Metafont and Metapost. I currently use TeXLive (a free win32 implementation) as my TeX system of choice, but have worked with emTeX (DOS, OS/2) for years and MiKTeX for a few years after that.
I work on development of the mfpic package If you are unfamiliar with mfpic, the idea of it is to use TeX commands to write .mf (or .mp) files, then input those to Metafont (or Metapost) to create figures. This is accomplished with the aid of a file of Metafont/post macros called grafbase.mf/mp.
The latest versions of mfpic are available for download.
mathdots.sty is a style file equally usable in LaTeX or plainTeX. It "corrects" the definitions of \vdots and \ddots, and adds an "inverse diagonal dots" command, \iddots, just like \ddots but on the opposite diagonal, lower left to upper right. The corrections include making the vertical dimensions scale along with the current math sizes, and taking the "dot" from the same math font as the math punctuation dot rather than the current text font. You can get it as a zip-file mathdots.zip here, or find it on CTAN.
mpsproof.tex is a drop-in replacement for mproof.tex which can be used to view proofs of MetaPost figures. mproof.tex is hard-wired to produce proofs only for tex+dvips, whereas mpsproof will do the same, but also allow the use of pdftex. You can get mpsproof.tex here. It is now a standard part of the MetaPost distribution, so if you need it you should already have it.
Splines provides MetaPost (or Metafont) macros for creating relaxed or closed cubic splines connecting a list of points. Also, for graphing a function from a list of pairs, using spline equations to interpolate between the points. You can download it in splines0_3.zip.
The tex file stmary.tex provides the stmaryrd symbols for plain TeX.
This consists of two main files: minifp.sty and mfpextra.tex. The first allows computations in TeX with fixed-point real numbers (the "fp" in the name) with not more than 8 digits to the left of the decimal and not more than 8 digits to the right (the "mini" in the name). The main binary operations of addition, subtraction, multiplication and division are provided, also max and min, and a number of simple unary operations.
The file mfpextra.tex supplies some extra computations: sin, cos, exp, log, sqrt, etc.
Download the zip file minifp.tds.zip, copy it to some texmf folder, and unzip it there.