Mophie Juice Pack for iPhone 3G/S: $8 at 1SaleADay.com (today only)
This thing cost roughly one arm and one leg when it first came out, if I recall correctly. Now it can be had for eight bucks. Make sure you’re still rocking an iPhone 3G or 3GS, though.
If so, you’ll enjoy a relatively sleek backup battery that doubles as a protective case. The deal is good today only, so hop to it.