The hardest part about using drupal's javascript features is knowing where to begin: they remained an arcane mystery to me until today. After drinking a bit of whiskey, and studying how core was using Drupal.behavior, I learned it couldn't be simpler. If you don't know how to do this, take a moment:
Konacha is a testing tool for JavaScript applications running on Rails.
A small tutorial on how to build a password strength meter like the one on Google's new account form.
The purpose of this book is to provide an overview of the jQuery JavaScript library; when you're done with the book, you should be able to complete basic tasks using jQuery, and have a solid basis from which to continue your learning. This book was designed as material to be used in a classroom setting, but you may find it useful for individual study.
JSLint is a JavaScript program that looks for problems in JavaScript programs. It is a code quality tool.
Modernizr adds classes to the <html> element which allow you to target specific browser functionality in your stylesheet. You don't actually need to write any Javascript to use it.