The not so Short Introduction to LaTeX - Perhaps the best place to start off, but also an indispensable reference
LaTeX Tutorials - A Primer - Another fantastic reference covering slightly different topics than above
Arbitrary LaTeX Reference - Good web reference for quick look-ups of forgotten syntax
FancyHdr CTAN page - The pdf maintained here is great when dealing with complex headers/footers in LaTeX
Importing graphics into LaTeX (PDF) - Reckdahl tells you all you need to know about graphics in LaTeX
Harvey Mudd College - Mathematics Department has produced a practical class for math-intensive homework
Winston Chang hosts a two-sided cheat sheet, particularly useful for quick lookup of common LaTeX commands
Dario Taraborelli has a wonderful collection of tips and trick, particularly regarding XeTeX
Ted Pavlic has developed many exceptional LaTeX templates, from résumé/CVs to lab reports
Reed College has prepared an exceptional archive of material for those writing in the humanities
IEEE Author Digital Tool Box provides all the necessary files to produce articles according to IEEE specifications.