The Codrops Playground has tons of experiments, plugins and articles related to web development and programming. Full of HTML, CSS and JS snippets.
Animista is a place where you can play with a collection of pre-made CSS animations, tweak them and get only those you will actually use.
30 Seconds of CSS
A curated collection of useful CSS code snippets you can understand in 30 seconds or less. A very useful tool for developers on the go.
CSS-Tricks Code Snippets
CSS-Tricks offers the most essential code snippets for HTML, CSS, JS, WordPress, PHP, Sass, jQuery and more.