Adobe Color
A very useful free color wheel and color palette generator for designers and developers. You can also upload an image to get colors from it.
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Grabient
A great tool that generates linear gradients, with the ability to customize each gradient, and download the CSS gradient code.
Dribbble Color
Dribbble Color is a wonderful tool that allows you to display all dribbble shots, based on certain colors.
Gradient by ShopFactory
Gradient by ShopFactory is a great tool for finding wonderful gradients, to use in your next project. It allows you to control the gradients based on different settings.
uiGradients
uiGradients is a community contributed collection of beautiful multi-color gradients, free to use on your next UI or design project.