Hypertext Help with LaTeX: Provide reference information for those with some familiarity with LaTeX

LaTeX on the CTAN: The macros/latex section of the Comprehensive TeX Archive Network. Most existing LaTeX packages are collected there

TeXnik: A collection of LaTeX tips and tricks

The Not So Short Introduction to LaTeX2e: LaTeX2e in 139 minutes by Tobias Oetiker. A short course on LaTeX