As a geek, I may not shy away from math, but in a store, holding a hot dress on sale with nothing but a price tag and a 15-percent-off sign hovering over it, I start to panic a little. "Quick, how much is that?!" I ask whomever I'm shopping with. For some reason, I need sale prices right now in order to make a decision.
Solving my panicky retail problem is the iPhone App onSale!, which has you enter the item's original price, the discount percentage, any additional discount, and even the tax so you get an accurate prediction of what you'll pay at the register.
This app costs a dollar, but will save you precious shopping time, and you can't put a price on that.