J., my favorite product for under-eye circles (also for blemishes of any kind, and even birthmarks) is a cover-stick called Physicians Formula (well, that's the brand I prefer, but other companies make this product too).
I should warn you that it's GREEN...which seems very odd, I realize, but of course it doesn't appear green after you rub it in (use sparingly!). Best of all, it's availabe at most drug stores and at Target -- so not expensive, and it lasts forever. I've tried this product on a lot of people in the course of producing film segments "in the field" (in the absence of make-up artists), and found it works with most skin tones, though I don't recommend it if you are African American -- if you are, I'd recommend the yellow cover stick suggested by another commenter. (then, after applying, just lightly brush your face with a compressed powder in your correct skin tone).
no more dark circles!