This page presents a collection of websites which I find guide useful and informative. I just add links as I come across sites by chance. I do no research about these topics.
Nice color palettes can be found at Flat UI Colors
The Stocks: a website with many links to images, videos, icons, colors and fonts
If you need some user data for test cases you can get random data from Random User Generator.
On the one hand, this website is outdated because it creates grids with floats, but on the other hand this site can still be useful for creating css for your fonts.
Play around with CSS Grid: http://cssgridgarden.com
Email testing can be done with: https://mailtrap.io/
Draw a polygon: https://bennettfeely.com/clippy/
Page Speed Insights Rules (from google): https://developers.google.com/speed/docs/insights/rules
which is part of: https://developers.google.com/web/fundamentals/
An article about anonymous web surfing from Bill Hess on Pixel Privacy
A list of styleguides can be found at: http://findguidelin.es/
I consider webflow worth to look at.