
The WordPress Help Sheet
The Advanced WordPress Help Sheet
ExpressionEngine
ExpressionEngine Quick Reference Chart
PHP
MYSQL
Ruby on Rails
JavaScript
jQuery 1.3 Cheat Sheet
Prototype 1.6.0.2 Cheat Sheet
XHTML
(X)HTML Elements and Attributes
CSS
ActionScript
Actionscript 2.0 Cheat Sheet Flashpaper
ActionScript 2.0 to 3.0 Migration Cheatsheets
Other resources
The people at Dzone have some more great cheat sheets for developers and designers. Check it out: RefCardz