tintui.com – A very useful color picker tool by João Piedade with all the famous UI colors in one place.
Colourcode is another one which offers a very original-yet-easy way of creating them.
The color schemes are generated with the mouse location/movements and multiple options exist for setting the scheme type: monochrome (with light/dark grey), analogic, triad and more.
Once created, it can be downloaded as styles (.less and .scss) or .png. It is also possible to save the palette for future use or share with a link.
Colourcode can be used freely online or, it is open source, it can be self-hosted.
iWantHue is a web-based color palette generator that provides powerful settings for finding the colors that play well with each other.
It works by defining a range of hue, chroma +lightness and the app can return a palette of any size instantly.
The palettes are provided in multiple formats (RGB or HEX, as a list or JSON) and, also, there are also presets to create faster.
COLOURlovers, an impressive website/community focused on colors, is sharing a handy tool named COPASO.
The tool is a web-based color palette generator for creating palettes and saving them for future use.
COPASO allows you to pick the colors from a color wheel or upload an image, optionally pixelate it, and grab from there.
There is a built-in search to find colors and palettes generated by the community with a sorting feature using various parameters.
P.S. Some features requires signing-in with a free membership.