How did I, online shopping Jedi, not know about Ebates?! One of my friends just turned me on to this website, which gives you both online coupons and cash back on your purchases from a ton of my favorite online stores.
For me that means Bluefly, Nordstrom, drugstore.com, as well as the Apple Store and eBags (a great destination for laptop bags).
Just sign up (it's free) and once signed in, navigate to the store you want to shop at from Ebates. Once you make a purchase, Ebates tracks it for you and sends you a check for your cash-back credit!
I've also heard good things about Fatwallet, which is similar to Ebates. Sometimes I think the universe wants me to shop.