WebGradients is a free collection of 180 linear gradients that you can use as content backdrops in any part of your website.

Trianglify Generator

Curated gradients for designers and developers

Copy Paste Color Pallette from Flat UI Theme

Color Claim was created in 2012 by Tobias van Schneider with the goal to collect & combine unique colors for my future projects.

Best Flat Colors For UI Design