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
web4college.com/css-play/index.php

Force CSS and JS changes to go live immediately
(Very useful when the browser is not showing your updates right away as your working on the site.)

Colors

Colors: material ui – colors and Adobe – Color Wheel and htmlcolorcodes.com
colorbox.io
cssfilters.co
coolors.co
flatuicolorpicker.com
qrohlf.com/trianglify
websitesetup.org/website-color-schemes
jenreviews.com/color-meaning

colorspark.app (Load random colors and gradients. Get inspired and download the CSS.)
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
cssmatic.com/box-shadow
css3buttongenerator.com
cssbuttons

sitepoint.com – css properties to control web typography
marketblog.envato.com/web-design/css3-typography-code-snippets

Paulund – Typing effect
h5bp github – Effeckt.css

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

bennettfeely.com/clippy

css-tricks.com/the-backdrop-filter-css-property

Grid
cssgrid.cc
cm.engineering/getting-to-know-css-grid-layout-818e43ca71a5
https://premium.wpmudev.org/blog/css-flexbox/
flexboxfroggy.com
learncssgrid.com
gridbyexample.com/examples/
alistapart.com/article/the-new-css-layout-excerpt
smashingmagazine.com/2018/05/future-of-web-design
medium.com/codyhouse/create-your-design-system-part-2-grid-layout-aa961d59b8d6

designmodo.com/tabbed-widgets

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

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

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

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

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

Animation
https://github.com/michalsnik/aos
https://daneden.github.io/animate.css/
https://animista.net/
3dtransforms.desandro.com
Upgrading CSS Animation With Motion Curves – smashingmagazine.
freefrontend.com/css-flip-cards

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
freefrontend.com/css-toggle-menus

Material design and style guides:
styleguides.io
getuikit.com
designsystem.digital.gov/components/form-controls
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
30-seconds.github.io/30-seconds-of-css

Additional resources:
spaceninja.com/2018/09/17/what-is-modular-css/

CSS Cheat sheet
10 fixed CSS backgrounds.