Sites

flexbox.io
WHAT THE FLEXBOX?! A simple, free 20 video course that will help you master CSS Flexbox!
Learn CSS Layout
This site teaches the CSS fundamentals that are used in any website’s layout

CSS-Tricks

CSS | MDN
CSS documentation at Mozilla Developer Network
Variable grid system
A quick way to generate an underlying CSS grid for your site.
WTF, HTML and CSS?
Reasons HTML and CSS might make you say what the fuck. A curated list of commonly frustrating HTML and CSS quandaries, miscues, and dilemmas. (by @mdo)
Awesome CSS
A curated list of CSS lists
CSS3Files
CSS Tips, Tricks and Tutorials
Must-Watch CSS
A useful list of must-watch talks about CSS
CSS protips
A collection of tips to help take your CSS skills pro
Maintenable CSS
Is an approach to writing modular, scalable and of course, maintainable CSS.
Flexbox Patterns
Build awesome user interfaces with CSS flexbox. Examples and source code included.
How to center in CSS
Centering in CSS is a pain in the ass. There seems to be a gazillion ways to do it, depending on a variety of factors. This consolidates them and gives you the code you need for each situation.
Grid by Example
A collection of usage examples for the CSS Grid Layout specification.

Tools

Frameworks

Bootstrap
(claims to be :) The most popular front-end framework for developing responsive, mobile first projects on the web.
Foundation
(claims to be :) The most advanced responsive front-end framework in the world.
Materialize
A modern responsive front-end framework based on Material Design.
Semantic UI
UI is the vocabulary of the web. Semantic empowers designers and developers by creating a language for sharing UI.
Flat UI
Free Bootstrap Theme.
Pure.css
A set of small, responsive CSS modules that you can use in every web project.
MinimalCSS
Minimal CSS Framework
Skeleton
A dead simple, responsive boilerplate.
Basscss
Low-Level CSS Toolkit
awsm.css
Simple CSS library for semantic HTML markup
Mobi.css
A lightweight, scalable, mobile-first css framework

Modules

Normalize.css
A modern, HTML5-ready alternative to CSS resets.
reset.css
Genuine Eric Meyer’s Reset CSS bundled as npm package.
Baloon.css
Simple tooltips made of pure CSS
radiobox.css
Tiny set of CSS3 animations meant for your radio inputs.

Preprocessors

Compass
Compass is an open-source CSS Authoring Framework
less
Leaner CSS
stylus
Expressive, dynamic, robust CSS
Sass
CSS with superpowers
PostCSS
Transformin CSS with JS plugins (IMHO, the most promising CSS preprocessor)

Grid systems

Toast
Insane, no-nonsense CSS grid

Bootstrap’s grid system

960 Grid System

Unsemantic
Unsemantic is a fluid grid system that is the successor to the 960 Grid System.
Jeet Grid System
A grid system for humans
SuperGiGi
Italian Grilleria

Animations

Animate.css
Just-add-water CSS animation
Hover.css
A collection of CSS3 powered hover effects to be applied to links, buttons, logos, SVG, featured images and so on.

Demo

CSS visualizer

Articles and other resources

About Flexbox

Nice collection of CSS animated loading indicators

CSS Development & Testing Tools

CSS Vocabulary

CSS Shapes Editor for Chrome

12 Little-Known CSS Facts

Get on the CSS Grid!

CSS shapes

CSS Diner - Where we feast on CSS Selectors!