Some very fine tutorials have been written, over the years. Michael Doob's splendid 'Gentle Introduction' to Plain TeX has been stable for a very long time. Another contender in the same game is from one D. R. Wilkins, available on the web at http://www.ntg.nl/doc/wilkins/pllong.pdf
More dynamic is Tobias Oetiker's '(Not so) Short Introduction to LaTeX2e', which is regularly updated, as people suggest better ways of explaining things, etc. The introduction has been translated into several languages other than its original English.
Harvey Greenberg's 'Simplified Introduction to LaTeX' was written for a lecture course, and is available on CTAN (in PostScript only, unfortunately).
Peter Flynn's 'Beginner's LaTeX' isn't yet (April 2002) complete, but is another pleasing read. It too started as course material.
The AMS publishes a "Short Math Guide for LaTeX", which is available (in several formats) via http://www.ams.org/tex/short-math-guide.html
Reference material is somewhat more difficult to come by. A sterling example is set by David Bausum's list of TeX primitives.
Some university departments make their local documentation available on the web. Most straightforwardly, there's the simple translation of existing documentation into HTML, for example the INFO documentation of the (La)TeX installation, of which a sample is the LaTeX documentation available at http://www.tac.dk/cgi-bin/info2www?(latex)
More ambitiously, some departments have enthusiastic documenters who make public record of their (La)TeX support. For example, Tim Love (of Cambridge University Engineering Department) maintains his deparment's excellent pages http://www-h.eng.cam.ac.uk/help/tpl/textprocessing/
Another summary by command (somewhat akin to David Basum's work for Plain TeX) is to be found at http://www.giss.nasa.gov/latex/ltx-2.html
People have long argued for (La)TeX books to be made available on the web, and Victor Eijkhout's excellent "TeX by Topic" (previously published by Addison-Wesley, but long out of print) was offered in this way at Christmas 2001. The book is currently available at http://www.eijkhout.net/tbt/; it's not a beginner's tutorial but it's a fine reference (contributions are invited, and the book is well worth the suggested contribution).
Norm Walsh's "Making TeX Work" is also available (free) on the Web, at http://makingtexwork.sourceforge.net/mtw/; the sources of the Web page are on CTAN.
The book was an excellent resource in its day, but is now somewhat dated; nevertheless, it still has its uses, and is a welcome addition to the list of on-line resources.