For web developer

Color Explorer is a free-to-use website which offers various tools to work with colors more efficiently.

You can upload an image to get the colors used in it automatically, browse color libraries (W3 named colors, web-safe colors, Focoltone, etc.), convert them between formats or make contrast analysis.

Optionally, and for free, you can sign-up as a registered member, create color palettes and save them for future use.

And, yes, there is definitely a cool color picker (which can also offer variations).

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.

ColorZilla is an impressive Firefox plugin that assists web developers and graphic designers with color related tasks.

A built-in eyedopper can get the color of any pixel and a color-picker helps playing with colors instantly. Picked colors can be added to the plugin's palette viewer/editor.

Also, generated or sampled colors can be auto-copied in CSS RGB, Hex and other formats which is handy to paste them into image editing apps.

A great feature is the ability to generate the color palette of any web page automatically and seeing where they are exactly used.

Beside these color-based functions, it can also measure between any to points

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.

Requirements: jQuery
Compatibility: All Major Browsers

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.).

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.

ColorSuckr is a free online service which can extract colors from an image & generate color schemes.

It can be used in 3 ways:

by mentioning the URL of the image
via a bookmarklet
or a Firefox add-on

Once the color schemes are ready, they can be downloaded as Adobe Swatch (.ASE) files or more schemes can be listed from COLOURlovers for any selected color.

ColorSuckr also offers the results in RSS, JSON & XML formats which makes an integration with any application possible.

Requirements: No Requirements

Pictaculous is a free web-based service which generates the color palette of an image you upload.

And, it provides color palette suggestions from Adobe Kuler & Colourlovers which can be used with the image.

Once the color palette is created, it can be downloaded as an Adobe Swatch file.

Pictaculous also enables you to send the image & get results via e-mail (which is nice for using the service in phones).

ColoRotate is a free web-based service that helps to work with colors in 3D, in real time & offers an unique experience.

Using the application, you can quickly see the multidimensional nature of colors and the relationships between colors.

With ColoRotate, you can create color palettes & save them. Its quick drag-and-drop interface to adjust hue, brightness & saturation helps you customize the colors with ease.

And, palettes created can be imported & exported in Adobe Swatch Exchange (ASE).

ColoRotate is also a color community where it is possible to browse & search themes created by other users.

P.S. You may also want to check "Learn About Color" menu of ColoRotate where it is possible to find valuable information on color like:

How do we perceive color?
Color models
& more

Requirements: No Requirements

Color Pallete Generator is a free web-based application that is creates the color palette of an uploaded image.

It supports .PNG, .GIF, .JPG, .JPEG images & creates 4 levels of palettes: light, medium, dark, complete.

The generator also prepares the related files for CSS & Photoshop swatches of the color palette created.

P.S. Up to 1 mb can be uploaded.