Gradient by ShopFactory is a great tool for finding wonderful gradients, to use in your next project. It allows you to manipulate the gradients based on color, pigment and lightness.
Color palettes generator, ColorSpace is a space for everything that has to do with color palettes and gradients.
uiGradients is a community contributed collection of beautiful multi-color gradients, free to use on your next UI or design project.
Color Hunt is a free and open platform for color inspiration with thousands of trendy hand-picked color palettes.