• https://coolors.co (generates up to five colours, has ads)
• https://hexcolor.co/palette-generator (nine colours)
• https://designs.ai/colors/color-wheel (five colours)
• https://colorkit.co/color-palette-generator
• https://www.rapidtables.com/web/color/color-wheel.html
• https://colors.lol - Overly descriptive color palettes. A fun way to discover interesting color combinations... Created by Adam Fuhrer.

Tools that take into account good visibility, colour-blindness, etc.?

• https://webaim.org/resources/contrastchecker WebAIM Contrast Checker

For scientific and technical visualisations specifically?