Here's a couple of my favorites!

ColourLOVERS is the go-to place for great color palette inspirations! IF you have trouble with color (like myself), give it a look see!

Formally called Pixelovely, Line of Action is great for a figure drawing session. There's specifics to what you want to draw and even a time limit setup as well as whether or not you want to draw clothed models or nudes.