A brief, but insightful, intro to color theory.
Coolors.co
An excellent and very intuitive app for generating color schemes, which you can save, download, share, or export in a number of formats including URL, PNG, or SCSS.
Click the spacebar, generate a new theme. Lock colors you want to keep, click the spacebar, and then the unlocked colors regenerate.
The app has a lot of subtle features, including the ability to generate shading, generate palettes from photos, or view your palettes after they’ve been adjusted to simulate eight different kinds of color blindness.
You can also explore palettes made by other members on the site (membership is free, too).
You can even get coolors as an iOS app or an Adobe Creative Cloud add-on.
ColorSupply.com
This is a great application. Like other tools on this list, it gives you palettes generated from your initial choice of color and color scheme, but then arranges them in icons, contrasting rows, gradients, etc.
A really nice related discussion, which great examples, can be found in this PDF file from DesignByNumbers.
Palettr.com
Make palettes from words. Type in a word, then Palettr will search an online database of photos and generate a number of palettes from the photos.
A fascinating utility.
Color.Adobe.com
One of the oldest color generation sites on the Internet. Very large community. Creative Cloud Integration. Similar to coolors.co.
DoneSmart.com Generator
Like a Photoshop color picker that automatically shows the corresponding colors in various color schemes. Basic interface but useful.
Paletton.com
Another long-time site. Very good layout preview feature. You can generate and edit palettes and then then them applied to layouts, artwork, and animations.
Colrd.com
Similar to color.adobe.com, but with the ability to generate larger palettes.
BrandColors.net
Official color codes for the world’s biggest brands.
ColorHexa.com
Input one color and see a range of info: conversions into various color models (rgb, hex, cmyk, lab, etc), some useful preview functions.
DinPattern.com
A great source of seamless background patterns
SubtlePatterns.com
And another.