Color palette tools

Theme Machine by Keith J. Grant is a combination of a color picker and a palette builder. It´s quick and comfortable. Keith has written an article on his blog: Theme Machine: A CSS Color Palette Tool.
leonardocolor.io allows to define color palettes and indicates WCAG contrast.
hihayk.github.io/scale/ allows to take a hex color and create lighter and darker colors with a beautiful and focused user interface. It does not indicate WCAG contrast. The tool is part of hayk.design.
huetone.ardov.me has sample palettes of Stripe, Chakra, IBM, GitHub and the like.
github.com/primer/prism/ is an experimantal GitHub tool for defining accessible color palettes to be used for light mode and dark mode. See the blog post Accelerating GitHub theme creation with color tooling.