Useful websites:

https://csszengarden.com
https://colours.neilorangepeel.com
https://colourpicker.com
https://specificity.keegan.st - specificity calculator
https://cssfontstack.com
https://unsplash.com - free pictures
https://fortawesome.github.io - icons and fonts
https://uigradients.com - nice gradients
https://www.programmableweb.com/ - a lot of APIs

Useful tools

https://www.npmjs.com/package/faker - TONS of fake data