Web Development kit

JavaScript

JavaScript Cheat Sheet

Addison-Wesley's JavaScript Reference Card

JavaScript Quick Reference

JavaScript and Browser Objects Quick Reference

JavaScript in 10 Minutes - Thanks Joseph

CSS

CSS Help Sheet

CSS Shorthand Guide

CSS Cheat Sheet

Cascading Style Cheat Sheet

CSS Cheat Sheet

CSS Quick Reference

Leslie Franke CSS Cheat Sheet

Design 215 CSS Quick Reference

CSS Level 1 Quick Reference

CSS Level 2 Quick Reference

CSS Property Index

HTML/XHTML

HTML Help Sheet

XHTML Cheat Sheet

HTML Cheat Sheet

HTML Character Entities Cheat Sheet

PDF HTML Cheat Sheet

Character Entity References in HTML 4 and XHTML 1.0

HTML & XHTML Cheat Sheet

HTML Tags

HTML Quick Reference Guide

A Simple Guide to HTML

Reference HTML Cheat Sheet

HTML Tags Cheat Sheet

AJAX

What’s Ajax? Cheat Sheet

Prototype Cheat Sheet

Scriptaculous Combination Effects Cheat Sheet

Scriptaculous Cheat Sheet - Thanks Joseph

AJAX for ASP.net Cheat Sheet

ASP.net AJAX Client Life-Cycle Events

MooTools Cheat Sheet - Thanks Joseph

Colors

RGB Hex Color Chart

Interactive Color Picker

HTML Color Codes

Color Reference Guide

Microformats

Microformats Helper Cheat Sheet

Microformats Cheat Sheet

Jack Daniel's Microformats Cheat Sheet

Browser Compatibility

W3C DOM Compatibility Tables

Browser Compatibility Interactive Table

XML

Fusebox 4.1 XML Cheat Sheet

VoiceXML Reference

MathML Reference

XML Schema 2001 Reference

XML Schema 2000/10

XSLT Quick References

XML TopicMaps 1.0 - Quick Reference Card

XML Quick References

XML Schema - Structures Quick Reference

XML Schema - Data Types Quick Reference

XSL FO Reference

XSLT Quick Reference Card

XSLT Reference

(thanks to all friends who helped me to compile a small list of web tools.)

Other tools:

http://www.plentyofcode.com/2007/07/programming-handbooks-and-cheatsheets.html

http://home.comcast.net/~shorvath/docs/rss_cheatsheet.pdf

Much more complete: http://www.fuzzyfuture.com/programming/the-developer-cheat-sheet-compilation/
Or: http://whatis.techtarget.com/definition/0,289893,sid9_gci826135,00.html
Or: http://lifehacker.com/software/cheat-sheets/
Or: http://www.bookmarkbliss.com/programming/the-developer-cheat-sheet-compilation/
Or: http://www.alvit.de/handbook/
Additional stuff on my blog: http://brianary.blogspot.com/2007/01/references.html
Not included in any of these lists: MIME types and extensions, RSS/Atom, HTTP(S), internationalization issues, section 508, SOAP, Unicode, language codes (ISO 639), country codes (ISO 3166), date formats (e.g. ISO 8601), currency codes (ISO 4217), etc.