Many people don't know that picking colors is as much science as it is art. Offline tools like color wheels have been around for years in order to help designers pick compatible colors. This article is a collection of resources that we use to aid us in selecting colors for client's websites.
HTML color palette generators
http://www.degraeve.com/color-palette/
Insanely useful tool that provides two distinct color palette for any picture you supply to the system.
Pros: Very easy to use, absolutely no learning curve.
Cons: Doesn't allow you to upload the image from your local computer; it only accepts an image's URL.
http://bighugelabs.com/flickr/colors.php
Same concept as the above tool.
Pros: Allows upload.
Cons: In my opinion the color scheme generated isn't as appealing as the first tool.
http://www.colr.org/
Again, amazing tool that allows the user to create a color scheme based on the image
Pros: Great for power users
Cons: Doesn't do enough automatically, leaves the last mile up to the user.
http://www.colourlovers.com/palettes
A community website and resource site which offers user-submitted color schemes.
Pros: This is more than a tool, but an entire website dedicated to colors
Cons: The search tool hardly ever returns results when based on an RGB code.
http://kuler.adobe.com/
Another user-contributed and rating model for pre-selected color palettes.
Pros: Uses the knowledge of crowds, so choices should have wide appeal.
Cons: Difficult to find a starting point for your search. After browsing for a few minutes all the pallette start looking the same.
http://www.colorjack.com/sphere/
Arguably the most powerful tool on this list because of the number of formulas it offers to find compatible colors.
Pros: Has more than one tool, explore the site for others
Cons: Doesn't allow direct hex code input to get started.
http://colorblender.com/
Supply the tool with one base color, and it provides you with complimentary shades.
Pros: Allows you to save your schemes in many different formats.
Cons: Pasting the color code in 3 separate boxes is tedious.
http://slayeroffice.com/tools/color_palette/
Same concept as the above but simpler.
Pros: Uncluttered interface
Cons: Requires a 3rd party tool to generate the base color hex code. Only provides shades of the same color.
http://www.colorschemer.com/online.html
Online version of a Windows based tool
Pros: The lighten and darken function is very useful and is typically requested by customers
Cons: The web based version has very limited functionality