Assuming you are addicted to shopping or willing to struggle through Chinese characters to be able to buy things you can't find in Beijing easily, this is the app for you.
Taobao is like a cross between Ebay (new stuff)/Amazon/flea market and the guy selling watches on the street corner in NYC, that's about the only way I can describe it. With the right Chinese characters you can find anything from cheese sticks to watches to shoes to Halloween costumes and more. Buyer beware it isn't the easiest site to navigate, I prefer the online version using Chrome so it will translate which can be a comedy show in and of itself. Also be aware that if is seems too good to be true, it probably is, last I heard fakes are abound on Taobao. I try to buy things that are a good deal but never too cheap. In China, price is a good indicator on quality, all the time.