I'm trying to improve my web development skills. I'm already familiar enough with HTTP, HTML, CSS, and Javascript, but I'd like to take advantage of some frameworks/helpers for site layout, CSS, and Javascript support.
What are the best HTML/CSS/Javascript web tools/frameworks out there? (In this question, I'm not looking for scripting-language-based frameworks like PHP, Ruby on Rails, or Django - but more "raw" HTML/CSS/Javascript frameworks.)
So far, I've done a quick look at things like YUI, jQuery, Prototype, Scriptaculous, and a few others, but I'm curious to know what people have had good experiences with.
One item per answer would be nice!