Here are links to two of the most important LaTeX resources on the web. The Comprehensive TeX Archive Network (CTAN) is an ftp site which houses an enormous collection of TeX packages, styles, macros and sample files. The TeX User's Group is a worldwide organization devoted to the use and promotion of TeX. For more general (La)TeX links and information, please visit the links page.
There are many ftp mirrors of the Comprehensive TeX Archive Network. However, Graham Williams' "TeX Catalogue Online" site lists links to many of the most common (and some not-so-common!) TeX packages and styles. He provides a graphical interface which makes searching for style files relatively painless. If you happen to stumble across a TeX file requiring a package or style that you don't have, CTAN Online is a good place the check.
CTAN Official Site
CTAN Catalogue Online
CTAN ftp Site
The best way to describe The TeX User's Group is in the group's own words:
The TeX Users Group (TUG) was founded in 1980 to provide leadership for users of TeX, Donald Knuth's revolutionary typesetting system. It represents the interests of TeX users worldwide: if you use TeX in any of its forms, please consider joining TUG.