If you own more than one Apple handheld product on the list below, this is the extended battery you'll want to have, but mostly in case of an emergency.
A juice pack is an extended battery that you can attach to a handheld device to increase its battery life. For the case of the iPhone 3G, you definitely want one. Most juice packs, such as the Juice Pack Air, are designed for a specific model of the iPhone or iPod. The iPWR SuperPack is the first we've had our hands on that works with multiple models, including iPhone, iPhone 3G, iPod Touch (first and second generation), iPod Video, iPod Nano (third and fourth generation), and iPod Classic. Plus, the sub-$60 price tag makes it the most affordable and versatile juice pack on the market.
In order to support multiple devices, the iPWR SuperPack doesn't come in a form-fitting form factor. Instead, it's just a little square box that can be attached to the port at the bottom of the device. The box has the same width and depth as the iPhone and when used with one adds about 50 percent more length to it. It's not a good idea to keep it plugged in at all times, since the iPhone's port isn't the most sturdy of connections, but the phone can be used just fine with the pack attached.
Even if you don't mind the extra length, you will still need to remove the juice pack when you want to sync the device with a computer, or when you want to charge either of them. The JuicePack's USB port is located near the same space by which it attaches to the phone, making it impossible to connect it to anything else at the same time, so you'll need to bring the pack's USB cable and the iPhone's or iPod's USB cable when traveling. Other juice packs like the Mophie Juice Pack allow you to charge and sync the phone without having to take them off.