I recently stumbled on colorsafe.co and really like it!
I'd already been using tools for checking contrast ratio to make sure my color choices pass WCAG accessibility standards. (Lea Verou's contrast-ratio.com is my longtime favorite.)
But what I like about colorsafe.co is that it gives you a quick click interface for choosing colors. I find it especially handy for quickly picking slide deck colors - because not only are high-contrast color combinations good for accessibility, they also make your slides look better on projector screens in bright rooms!