Colors is a standalone JavaScript library that comes with features to manipulate colors in multiple ways.

The library can handle multiple types of unit conversions including RGB to a Hexadecimal and Hexadecimal to RGB or HSV.

It can get the complimentary color of any Hexadecimal or RGB value and can generate random color values.

P.S. The library is still under heavy development and in beta phase.

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.

Coloreminder is a free web tool that aims to help web designers find the variations of any given color quickly.

It displays a list of colors where you can choose from or you can simply use the color-picker to select (paste) a color.

Once selected and submitted, the website displays you the variations and other color options (invert, greyscale, complementary, etc.).

Color Scheme Designer is a web-based application that provides a high quality experience for creating color schemes online.

The color wheel lets users to define a scheme based on a single color or more by adding more controls to it.

With 15+ presets, ability to control the saturation/brightness & contrast levels, it is possible to switch between variaties of the color schemes created.

A great feature is the ability to preview the colors used on different outputs like a scheme or a webpage preview.

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.