If it's scheme [wikipedia.org] you're looking for, there's R5RS [schemers.org] and the SRFIs [schemers.org]; also, don't forget the world's best-written programming book: SICP [mit.edu].

Quick Ref [quickref.org]
QuickRef currently includes: C, C++, CSS, HTML, HTML DOM, Java, JavaScript, MySQL, Perl, PHP, Python, and Ruby

http://www.regular-expressions.info/ [regular-expressions.info]

cpan.org, Perlmonks.org perldoc.perl.org [perl.org]
use Perl; [perl.org] comes high on the list, along with PerlMonks [perlmonks.org] and PerlBuzz [perlbuzz.com].
Perl.org [perl.org] in general gets points for being where you can find Use, perldoc, and more.

Another good one would be the Python Cookbook [activestate.com] on ASPN. And at a more abstract level, Ward's Wiki [c2.com] has explanations and discussions of a wide range of interesting tools, languages, and techniques.

python.org

For java, you have to master the API [sun.com].http://java.sun.com/reference/ [sun.com]
There is also the Java Forums [sun.com] which is a great place to ask people to do your intro CS homework and get flamed.
I agree. I have been using Java for a while now and have always found the information I needed at java.sun.com [sun.com]. Just about anything you could want to know about Java can be found in the reference section [sun.com]. The API section [sun.com] has all of the classes listed alphabetically for the more recent versions of Java with a fairly detailed description of each class and its methods.

php: php.net
coldfusion: adobe.com
html, css, javascript, sql: w3schools.com
python: python.org
c/c++: devshed.com
java: java.sun.com

For JavaScript I head over to

developer.mozilla.org

It's pretty clear, but unfortunately you still have to watch out for browser incompatibilities. They sometimes will have a note about it though.

One of the best tutorials and references for Tcl is the Wikibook [wikibooks.org] on Tcl programming.

Object Pascal reference [freepascal.org] & C reference [uiuc.edu].

http://cprogramming.com/ [cprogramming.com] - best site for beginners in my opinion.
www.cplusplus.com
Beats the hell out of man pages for the POSIX C libraries.
Standard Template Library:
SGI reference [sgi.com] - readable, comes with informative explanations of concepts
libstdc++ reference [gnu.org] - less readable, but it may have some non-standard things that aren't in the SGI docs, useful if you're using libstd++
Boost libraries [boost.org]:
For anything not present in the standard library, these are the next place to check. They're freely usable in commercial projects.
The STL reference [sgi.com]. Please keep this page open when you are writing your C++....
C++ FAQ Lite [parashift.com] is an excellent site for C++ information.

Perhaps this a question for Stack Overflow [stackoverflow.com]?

Aside from the obvious [haskell.org], there are some interesting papers [microsoft.com], essential reading [microsoft.com], a mailing list [haskell.org], a tutorial [haskell.org], and even a (reasonably complete) wikibook [wikibooks.org].

http://www.gotapi.com/ [gotapi.com] It's got all the good reference sites in one. You click the reference site, it adds a tab to the gotAPI webpage. It has a really good search box. No signup required. Best all-in-one reference ever.

SNOBOL - http://www.snobol4.org/ [snobol4.org]
Logo - http://el.media.mit.edu/Logo-foundation/logo/programming.html [mit.edu]
FORTH - http://www.phact.org/e/forth.htm [phact.org]
Prolog - http://www.logic.at/prolog/faq/faq.html [logic.at]
Algol - http://www.algol68.org/ [algol68.org]
Lucid - http://www.haskell.org/haskellwiki/Lucid [haskell.org]
PL/I - http://www.users.bigpond.com/robin_v/resource.htm [bigpond.com]

Kinda obvious (no more obvious than php.net being the best reference for php I suppose), but: http://docs.jquery.com/ [jquery.com]

For Ruby as well as Ruby on Rails, I like http://apidock.com/ [apidock.com]. It's the first Ruby/Rails doc site that I've seen that has the ability to contribute, rate contributions, etc.

SGI's STL site [sgi.com] is excellent, though I'm not not sure how up-to-date it is.
The C++ Programming Language [amazon.com], by Bjarne Stroustrup, is the only essential and authoritative language reference, other than the standard itself. It isn't a web site, but programming in C++ isn't something you pick up on a whim.
And of course Boost [boost.org], the first place to look when you think, "There should be a library for this."

Lua:
Official website: http://www.lua.org/ [lua.org]
Direct link to manual: http://www.lua.org/manual/5.1/ [lua.org]
Lua community: http://lua-users.org/wiki/ [lua-users.org]

Wikibooks [wikibooks.org] because if it isn't already there, someone will eventually write one and make it open sourced.
I invite Slashdot readers and posters to write their own ebooks at Wikibooks in an open source license.

Personally, I can only vouch for the content on one site: Free Java Lectures: Two Semesters of College-Level Java for free [googlepages.com].

developer.apple.com [apple.com] - Apple documentation, good reference but sometimes lacking in details
cocoadev.com [cocoadev.com] - Cocoa wiki
cocoabuilder.com [cocoabuilder.com] - Cocoa mailing list archives

For Java: JDocs.com [jdocs.com].