Spectrum is a jQuery colorpicker plugin that probably comes with all the features you are looking for.

It is image-free (only CSS and JS) and can be used for all browsers or only as a polyfill for the input[type=color] of HTML5 .

The picker is actually familiar to designers/developers as it is the same one that comes with WebKit DevTools (created by the same developer) but now available as a jQuery plugin.

It has support for color palettes by storing/displaying the colors you chose, the default selected color can be set or it can be configured to only display the palette with pre-defined colors.

Alpha transparency selection is available, can be always visible (without clicking to a picker icon) and supports multiple color formats.

Spectrum can be customized with CSS and it works on mobile as well.

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.

iWantHue is open source and anyone can use it to create palettes with only JavaScript.

Colllor is a free-to-use web application that helps us to create a more consistent color scheme.

For any given color, the app displays the exact value of darker shades of it (not something that looks darker but the ones that derive from the same color where all have a common denominator sharing hue, lightness or saturation values).

Using Colllor, we can easily explore tints, shades, tones or similar colors and mix colors to create new ones.

It understands both the HEX values and color names. Also, you can instantly pass the color name to the URL (like http://colllor.com/blue) to get started quickly.

P.S. There is also a Chrome application for it.

xcolor is a jQuery plugin which allows you to perform many different manipulations on colors.

First of all, it can parse and convert colors between different formats with methods like getRGB(), getHSV() or getHEX().

Another powerful feature is extracting or combining colors (like red + blue would result in purple).

Also, it can generate complementary colors, lighten/darken them, find analogous/monochromatic colors of an item and much more.

The plugin comes with other functions where the most interesting one is: isReadable which performs a readability test for a text on a given background.

Tip: To perform the same actions on the server-side, the author also has a PHP class with very similar features.

A web designer usually starts with a color and needs the variations of that color for hover, borders, gradients, etc. while designing/styling an element.

In order to be stable for the overall of the design, while choosing color variants, picking them from the same levels is a much better way compared to just guessing using the color picker of our image editing software.

0to255 is a simple-yet-effective web-based color finder which is specially created for web designers to make this process easier.

After picking the color you want to start with, the tool provides you a range of colors from black to white using an interval optimized for web design.

And, just click the variation you want to use and the hex code is automatically copied to your clipboard.

Designers, who wonder the RGB of any color that is around their mouse can get the results instantly.

Small utility named Pixie, runs on your system tray, you can call it with a click and it will show you HEX, RGB, CMYK & HSV values of any pixel you’re pointing.

Many designers take a screen capture, run their favorite image editing software like Photoshop, Fireworks and paste the capture to the image editing software to get the image color values. With Pixie, no need for these steps.

An extra feature of Pixie is, it shows the x y position of the mouse pointer.