Kensington K33457US Charging Dock with Mini Battery Pack for iPhone and iPod: $49.18 Shipped

Let's face it...the iPhone's battery life SUCKS. Unless you are recharging it, it won't last the day under normal use. By normal I mean playing a few games of Madden 2010 football. You need an extra battery. Kensington thinks they have you covered.
As noted by the December issue of MacWorld, this product has an annoyingly long name: Charging Dock with Mini Battery Pack for iPhone and iPod, but they liked it nonetheless. It connects to your computer via an included USB cable, letting you dock, charge and sync your iphone or ipod. When you are ready to go, grab the battery, and when your iPhone starts to die, plug it into the iPhone's dock connector. It has LED lights that show the remaining charge. It's only a 1000mAH battery, as compared to other external batteries that are 1800mAH, but it works. The normal price is $69.99. At $49.18, it's cheaper than buying a plain dock from Apple, and you get the convenience of an extra battery.