Design

Accessibility

Vischeck: Simulate how your images look for people with different forms of colorblindness (web-based)
Color Oracle: Simulate how your images look for people with different forms of colorblindness (desktop-based, more types of colorblindness)

Adobe Color: Create, share, and explore rule-based and custom color palettes.
ColourLovers: Like Facebook for color palettes.
Coolors: Generate random palettes that look great.
viridis: Perceptually uniform color scales.
Scientific Colour-Maps: Perceptually uniform color scales like viridis. Use them in R with {scico}.
ColorBrewer: Sequential, diverging, and qualitative color palettes that take accessibility into account.
CARTOColors: More sequential, diverging, and qualitative color palettes that take accessibility into account. Use them with {rcartocolor}.
Paletteer: The {paletteer} package for R includes hundreds of different palettes.
HCL palettes: The {colorspace} package for R includes a ton of qualitative, sequential, and diverging HCL (hue-chroma-luminance) palettes that use fancy mathematical rules that maintain perceptual distance
Colorgorical: Create color palettes based on fancy mathematical rules for perceptual distance.
Colorpicker for data: More fancy mathematical rules for color palettes (explanation).
iWantHue: Yet another perceptual distance-based color palette builder.
Photochrome: Word-based color palettes.
PolicyViz Design Color Tools: Large collection of useful color resources

Google Fonts: Huge collection of free, well-made fonts.
fontpair: A directory of all sorts of Google Fonts pairings (for when you’re looking for good contrasting or complementary fonts)
The Ultimate Collection of Google Font Pairings: Another list of great, well-designed font pairings
“Pairing typefaces”: Google Fonts’s own guidelines for finding good pairings
Font Mixer: An online app for finding and testing different font combinations