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.
The Codrops Playground has tons of experiments, plugins and articles related to web development and programming. Full of HTML, CSS and JS snippets.
CSS-Tricks Code Snippets
CSS-Tricks offers the most essential code snippets for HTML, CSS, JS, WordPress, PHP, Sass, jQuery and more.
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.