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)

Colors

Adobe Color: Create, share, and explore rule-based and custom color palettes.
ColourLovers: Like Facebook for color palettes.
viridis: Percetually 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.
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 pallettes.
PolicyViz Design Color Tools: Large collection of useful color resources

Google Fonts: Huge collection of free, well-made fonts.
The Ultimate Collection of Google Font Pairings: A list of great, well-designed font pairings from all those fonts hosted by Google (for when you’re looking for good contrasting or complementary fonts).