Colr is a neat little app to let you play around with color and color schemes based on tags. Type in “fresh” and see what schemes come up, just like in flickr.
Well-Styled is another one of these useful color-wheels, allowing you to mix and match background plus four other colors.
DarkEye was born in June 2005. Beginning life as several experiments in .NET / AJAX, it allows you to specify colors and pull out all the sites using these colors.