There are a ton of free cheatsheets, quick references, and downloadable resources for programming languages and related technologies online -- in this post I've tried to organize and list some of the best for web development.
Meta-reference sites -- organized lists of references and cheatsheets:
http://www.gotapi.com/
A wonderful online API reference covering many programming languages.
http://www.digilife.be/quickreferences/quickrefs.htm
An organized list of quick references and cheatsheets.
http://techcheatsheets.com/
A great site with links to cheatsheets on lots of programming languages.
http://refcards.com/
Another great site organizing various cheatsheets.
http://whatis.techtarget.com/definition/0,...i826135,00.html
A very long and thorough set of links to cheatsheets all over the web.
Other meta-reference sites:
http://mypage.bluewin.ch/yuppi/links/cheatsheets.html
http://www.petefreitag.com/item/455.cfm
http://www.smashingmagazine.com/2006/10/30...css-latex-ruby/
http://www.fuzzyfuture.com/programming/the...et-compilation/
http://lorelle.wordpress.com/2005/10/10/ht...e-cheat-sheets/
http://3spots.blogspot.com/2005/10/cheatsheets.html
http://webdeveloper.econsultant.com/cheat-sheets/
Key providers of cheatsheets:
http://www.visibone.com/
Visibone, the company that makes the best commercial cheatsheets for HTML, CSS, colors, etc. Site includes free resources and downloadable versions of several great cheatsheets.
http://www.ilovejackdaniels.com/cheat-sheets/
Original cheatsheets for HTML, CSS, PHP, and other technologies.
http://www.deepx.com/resources/quickref/
Original cheatsheets for CSS, XML, and related technologies.
HTML cheatsheets:
http://html-tags.info/
Free version of Visibone's HTML cheatsheet.
http://www.ucc.ie/doc/World-Wide_Web/htmlcard.html
HTML cheatsheet from Andrew Ford.
http://www.utoronto.ca/ian/books/extras/html-7nov2000.pdf
HTML cheatsheet from Ian Graham.
http://www.ilovejackdaniels.com/cheat-shee...ml-cheat-sheet/
HTML cheatsheet from ILoveJackDaniels.
http://gosquared.com/liquidicity/archives/51
HTML cheatsheet from Liquidcity.
http://www.htmlgoodies.com/beyond/referenc...cle.php/3472851
Maran Wilson's HTML cheatsheet.
http://www.cdburnerxp.se/htmlcheatsheet.pdf
Florian Schmitz's HTML cheatsheet.
http://www.arnspub.com/QuickRef/
An online reference to HTML organized by tag.
http://www.blooberry.com/indexdot/html/index.html
Brian Wilson's online HTML reference.
http://www.ilovejackdaniels.com/cheat-shee...es-cheat-sheet/
ILoveJackDaniels' HTML character entities cheatsheet.
CSS cheatsheets:
http://www.deepx.com/resources/quickref/CSS-1.0.pdf
CSS cheatsheet from DeepX.
http://www.utoronto.ca/ian/books/extras/cs...et-7nov2000.pdf
CSS cheatsheet from Ian Graham.
http://www.ilovejackdaniels.com/cheat-shee...ss-cheat-sheet/
CSS cheatsheet from ILoveJackDaniels. (And really, who doesn't?)
http://www.gosquared.com/liquidicity/archives/33
CSS cheatsheet from Liquidcity.
http://www.spectrum-research.com/V2/projec...s_reference.asp
CSS cheatsheet from Spectrum Research.
http://www.veign.com/downloads/guides/qrg0007.pdf
CSS cheatsheet from Veign.
http://home.tampabay.rr.com/bmerkey/cheatsheet.htm
CSS cheatsheet from Brett Merkey.
http://lesliefranke.com/files/reference/csscheatsheet.html
CSS cheatsheet from Leslie Franke.
http://www.lazycat.org/webdesign/cssref.php
CSS cheatsheet from Matt Robinson.
http://www.blooberry.com/indexdot/css/index.html
Online CSS reference from Brian Wilson.
http://meyerweb.com/eric/css/references/css1ref.html
http://meyerweb.com/eric/css/references/css2ref.html
CSS online references from Eric Meyer.
http://www.design215.com/toolbox/css_guide.php
A detailed overview of CSS from 2006 that covers features that work on most browsers.
Javascript cheatsheets:
http://wps.aw.com/wps/media/objects/2234/2...refererence.pdf
Addison-Wesley Javascript cheatsheet.
http://sage.math.washington.edu/home/agc/l...tcheatsheet.pdf
Javascript cheatsheet from Holmer Hemsen.
http://www.ilovejackdaniels.com/cheat-shee...pt-cheat-sheet/
Javascript cheatsheet from ILoveJackDaniels. (I really don't understand how he produces so many cheatsheets, drinking as much Jack as he must.)
http://www.mytechsupport.ca/tools/docs/Jav...k_Reference.pdf
Luke Terheyden's Javascript cheatsheet.
http://javascript-reference.info/
Visibone's free Javascript reference card.
http://www.visibone.com/regular-expressions/
Visibone's free Javascript regular expressions card, focused on Javascript.
http://www.quirksmode.org/js/contents.html
Extensive assortment of Javascript references on various topics.
Browser compatibility matrices:
http://www.quirksmode.org/dom/compatibility.html
http://www.quirksmode.org/css/contents.html
Comprehensive browser compatibility matrices for DOM, HTML, CSS, and event handling.
Colors:
http://www.visibone.com/colorlab/
Visibone's free interactive web color picker.
http://html-color-codes.com/
Visibone's free web color quick reference guide.
http://www.veign.com/downloads/guides/qrg0006.pdf
Veign's web color reference guide.
PHP cheatsheets:
http://www.ilovejackdaniels.com/cheat-shee...hp-cheat-sheet/
PHP cheatsheet from ILoveJackDaniels.
http://www.stevengould.org/portfolio/indep.../PHPRefCard.pdf
PHP 4 reference card from Steven Gould.
BONUS -- Google cheatsheets:
http://www.google.com/help/cheatsheet.html
Official Google cheatsheet.
http://www.googleguide.com/advanced_operators_reference.html
Google advanced operators cheatsheet from Google Guide.
BONUS -- free downloadable programming books:
http://freecomputerbooks.com/
Outstanding site organizing links to many free programming books, tutorials, and lecture notes.
http://www.freetechbooks.com/
Another organized set of links to free programming books.
Other sites for free programming books:
http://2020ok.com/5.htm
http://www.techtoolblog.com/archives/195-f...ogramming-books
http://www.computer-books.us/
http://www.onlinecomputerbooks.com/
http://kickjava.com/books/
Source: http://blog.pmarca.com/programming/index.html