Getting Started with LaTeX
http://www.maths.tcd.ie/~dwilkins/LaTeXPrimer/
This is one of the first webpages I used as a reference for teaching myself LaTeX. I found it to be a good introductory reference, with the ability to quickly jump between sections and get good information about the more often used aspects of LaTeX.
The Not So Short Introduction to LaTeX 2e
http://www.ctan.org/tex-archive/info/lshort/english/lshort.pdf
After working with the above site for a while, I stumbled across this PDF file and have been using it as my main reference for years now. I have found this to be a great reference for the level of typesetting I have had to do so far. I also like that I can have it open in a seperate window while I am typing up my .tex file, and if I need to look anything up, I can quickly switch to that window and do a '<ctrl>-f' to find what I need.
The author updates the file with more information here and there, so if you use this, check back every now and then to see if any updates have been done.
Math Into LaTeX (3rd Ed.) - George Grätzer
http://www.amazon.com/Math-Into-LaTeX-George-Gr%C3%A4tzer/dp/
0817641319/sr=8-1/qid=1163208244/ref=pd_bbs_sr_1/102-6179061
-2654556?ie=UTF8&s=books
After using LaTeX for a few years, I decided to purchase an actual book on the topic. I ended up going with this one, and have found it to be quite good. The book provides more in-depth information on various aspects of LaTeX, while keeping it all accessible enough to make for a great reference.
Scott Pakin's LaTeX Stuff (The Comprehensive LaTeX Symbol List)
Main Page:
http://www.pakin.org/~scott/latex-stuff.html
The Comprehensive LaTeX Symbol List:
http://www.ctan.org/tex-archive/info/symbols/comprehensive/
Scott Pakin has put together all sorts of LaTeX stuff, and offers links to, as well as short descriptions of, each of those items (see Main Page). In particular, he has put together quite a list of (thousands of) symbols one may access in LaTeX (see link for comprehensive list). Quite a nice reference for learning how to typeset the more obscure symbols in Mathematics.
Note: After following the link for the list, I found it a lot easier to 'View' the .pdf and then save that, than it is to 'Download' it (and meander through the various directories on some of the mirrors).
Thanks to Sherod for letting me know about this .pdf/author