Amazon.com offers periodic online coupons on products ranging from groceries to personal care items, baby needs, electronics, and books when you purchase directly from Amazon (as opposed to third parties). Simply go to here to find a list of current available coupons, click "Clip this coupon" underneath the listed price of your chosen item, and the discount applies at checkout.
Drugstore.com has a similar program for online coupon clipping. Select a participating product from this list, add it to your bag, click on "Buy checked items" at the bottom of the page, and your coupon will apply automatically. Specifics about expiration dates and coupon limits are also listed at the bottom of the page.