There are a lot of great resources to help you create better CSS code.

I will add on to this article when I find additional resources that I find useful.

cssreference.io
learnlayout.com
w3schools.com: css

Colors

Colors: material ui – colors and Adobe – Color Wheel and htmlcolorcodes.com
cssfilters.co
coolors.co
flatuicolorpicker.com
qrohlf.com/trianglify

canva.com/colors
grabient.com
Colorzilla – Gradient Editor
gradientbuttons.colorion.co
gradienty – A very interesting way to add a gradient
webhostingplanguide.com – browse a CSS gradient list

CSSmatic – Border radius
CSS3gen – Box Shadow
sitepoint.com – css properties to control web typography
marketblog.envato.com/web-design/css3-typography-code-snippets

Paulund – Typing effect
h5bp github – Effeckt.css
cssbuttons
codepen – Progress bar
codepen – Tool tips
microtip

hacks.mozilla.org/2017/06/css-shapes-clipping-and-masking/
alligator.io/css/exploring-blend-modes

.webdesignerhut – pure-css-modal-window-effect

Grid
cssgrid.cc
cm.engineering/getting-to-know-css-grid-layout-818e43ca71a5
https://premium.wpmudev.org/blog/css-flexbox/
flexboxfroggy.com
learncssgrid.com

Image effects and image hover effects
http://html5-demos.appspot.com/static/css/filters/index.html

http://imagehover.io/
http://ianlunn.github.io/Hover/
http://tympanus.net/codrops/2016/11/23/tilt-hover-effects/
https://tympanus.net/codrops/2017/03/15/stack-motion-hover-effects

http://css3.bradshawenterprises.com/transitions/
alligator.io/css/cropping-images-object-fit

Hover over buttons and create an icon effect:
http://codepen.io/daviddarnes/pen/VLXxMa

http://www.hostingreviewbox.com/tools/css3-button-generator/
https://premium.wpmudev.org/blog/css-button-libraries-collections/

Animation
https://daneden.github.io/animate.css/
http://animista.net/
Upgrading CSS Animation With Motion Curves – smashingmagazine.

Picks up colors from web sites with stylifyme.

CSS Layerstyles
CSS 3 Generator

CSS Values
CSS Desk
CSS Lint

Navigation:
CSS multi level accordion menu
jonsuh: hamburgers
freefrontend: css-menu

Material design:
designyourway – material design resources for designers

Plugins:
csshero.org: CSS Hero
themeover.com: Microthemer

Long lists of great resources to go through:
designyourway – the most popular css libraries that are on github
Simplilearn – css3 resources ultimate list article
22 Essential CSS Recipes