I've started using Covergirl's Advance Radiance under eye concealer, and it has really helped reduce the appearance of my dark circles. You need to use it sparingly, though, to avoid looking jaundiced!.
Advanced Radiance Age-Defying Concealer :: CoverGirl
Someone mentioned lack of iron as a potential cause - that's interesting, I wonder if that might be my issue? I developed dark circles for the first time in my life last year when I went on a higher-protein, less-carb diet. It really helped with losing weight, and seems healthy enough, but I often wonder if I'm not getting enough iron. I don't eat meat, so my iron was typically from lots of leafy green veggies. Now that I eat more protein (fish, dairy) I seem to be eating less leafy veggies. Hmmm.
Any way to test iron levels without a dreaded needle?