The color schemer is an online utility, that besides generating a color scheme for your site, allows you to input a color and lighten or darken it. What a great utility!
And one of the more robust color schema generators is colorpicker. The color sliders and export options make it easy to use and quite flexible.
I've also started using the one at http://wellstyled.com/tools/colorscheme2/index-en.html. It shows you want the colors would look like for a person with color blindness.