So called Material Design is a design framework developed and maintained by Google. It describes UI/UX principles and graphical design recommendations for mobile and web apps. Combines the classic principles of successful design along with innovation and technology. Here you can find a collection of useful and really helpful resources on the topic.
Official Google’s Guidelines
An interactive web app for picking recommended Material Design colors and their matches.
Showcase of best practices about MD, inspirational examples of MD usage and downloadable MD assets.
Material Design icons
Interactive reference with official MD icons.
Lots of useful tools and assets.
An outstanding CSS framework which implements MD.
UI Component framework for Angular JS.