This page gives you a summary of most useful online resources for emacs known to me. (note: the web is huge. So, this is only ones i know of.)
If you want a systematic approach, best are the official docs and commercial books. See: Emacs Lisp References.
Note that emacs has many commands to search the bundled doc. See: Tips on Long Term Emacs Productivity ◇ Emacs: Finding Functions and Documentation Lookup.
emacswiki.org is a emacs wiki. Most useful info there are about hundreds of modes in the wild.
If you want to know what does a mode do, or if a mode exist for language xyz, this is the best resource. For example, it would list several modes for twitter, blogging interface, templates …, where to download, how to install, and usually has tips about the good/bad/tips of each.
The info on the site is often chaotic and not well organized, but still it is a great resource. Emacswiki.org is started by Alex Schroeder.
There are quite a few emacs blogs. They are useful for casual, periodic reading. Getting tips, latest news, trends. The most consistently tutorial-oriented known to me are:
I also run a emacs blog. Please subscribe at: Xah Emacs Blog.
www.reddit.com is reddit's emacs link collection.
planet.emacsen.org is a blog aggregate of about 80 emacs blogs. Usually random news, tips, code, in varying quality, and sometimes doesn't have that much to do with emacs.
There are many twitter micro blogs on emacs. As of 2012-01-27, the active ones with at least few tweets per week are:
I also run one, at: @ErgoEmacs and ErgoEmacs g+.
If you have a emacs or elisp question, the best place is online forum. Here's the top few good ones.
stackoverflow.com emacs is pretty good. Probably should be your first stop.
gnu.emacs.help newsgroup is FSF's official emacs help forum. The forum is read by many emacs developers.
FSF's GNU Emacs Dev Mailing List @ lists.gnu.org…. This is the most hardcore. If you know elisp well and really have a hard question, this is the best place to get answers.
For those who reads chinese, there's a very popular chinese emacs blog/wiki @ emacser.com. It's based in China, mostly used by Chinese people there. The site has lots of info, tips, discussions, including links to major Chinese elisp hacker's sites.
Emacser is started by ahei.info.
(Thanks to fangzhzh.info for a correction.)