You probably have this first one already. It's an adapter for charging in your car. Whether you're in your car when your phone dies, or you're at home during a power outage, your car's battery is a reliable power source to tap into.
Sometimes these are wired for a specific kind of phone or connection. That's fine -- but if you can find one that uses a generic USB socket, you'll be able to power a wider range of devices.
The model shown here, the Trans4m Multi Power Source 4 Way, has the added advantage of allowing you to recharge your phone from a pair of AS batteries -- which brings us to our next option...
Another relatively cheap solution you can keep at home or in your glove compartment are these battery-powered chargers. The one shown here is the Rayovac Turbo Charge. It can be found for under $10, it takes two AA batteries, and it gives you a few charges. It even offers a little LED flashlight. I figure that even if the zombie apocalypse takes out the power grid, I could still loot batteries from the local drug store.
If you're away from civilization or just want a renewable way to charge your phone, you could go solar. Portable solar chargers range in price from $20 to $80. Typically, these have a built-in battery that you can charge during the day and use when you want.
The model shown here is the Revolve XEMini Plus. It falls on the high end of the price spectrum, but its eco-friendly construction and broad range of features are a rarity.
But, if you're in a pinch, it's nighttime, and your solar charger hasn't seen the sun in a while, that's where good old-fashioned elbow grease comes in handy. Hand-crank chargers like the Slive-88 make an obnoxious sound and they take a lot of effort to make just a little power. Still, you can't go wrong having one of these in an emergency. If you can find one like this that integrates a radio and a flashlight, all the better.
Now if you're thinking more along the lines of a serious emergency kit for your car or home, companies like Duracell and Energizer make heavy-duty battery units that can deliver power for an extended period of time. The model shown here, the Duracell Powerpack 450 Jumpstarter, includes two standard power sockets and also offers a standard USB port.
Whether you're taking your gadgets camping or planning for a post-apocalyptic Road Warrior future, you can't go wrong with a solar panel backpack. There are several models on the market, including this one, the Voltaic Backpack. With it, you stay off the grid, while staying in touch.
Extended battery packs like the PowerMat Dual 1850 Rechargeable Backup Battery get an honorable mention in this list. They work great if you find yourself needing a regular infusion of battery life day-to-day, but they're not my first choice in an emergency. If there's no power source nearby, there's no way to recharge them, and all you have is an expensive skipping stone.