What's a good supplementary power source for an iphone?

My wife has the Splash O2 extended battery pack for the iPhone 4 and it works really well. As a bonus it's less expensive than some of the other battery packs on the market, and it seems quite well made. I use a Splash case as well (no battery) and it's my favorite iPhone case.

Tekkeon TekCharge. It uses AA batteries that can be found anywhere and isn't a single-use tool since it can charge just about any gadget. (Note, it does NOT have a sufficient charge for an ipad, but works great on an iphone 3GS). Works really well to keep an iphone charged during a multi-day camping trip. I use it with eneloop rechargable batteries and it's been going strong for about 2 years now.

I really enjoy the Mophie case on my 3GS. They have the other iPhone models as well. It provides double the power, plus protection. I don't mind the added heft, as I dropped the phone too much previously, hence the need for the case protection as well.

I did not want a charger that required a case (like the Mophie), since I like to 'set it and forget it' with my phone case.

I did not want a charger that did not attach securely, or made it difficult to use my phone while charging. There are many cheaper chargers that keep coming out of the phone unless you just lay it down on a table.

I went for the Just Mobile Gum iphone charger, which took my phone through a 14 inning baseball game with power to spare. I picked up a short sync cord, as well as a retractable one, to connect the phone to the battery. My only complaint is that I wish the charger recharged faster, you need a good couple of hours before it's back up to full speed. Recharge via USB.

You'll be wanting to look at Milwaukee Power Tools' M12 Cordless Power Port 2349-20. It uses the M12 compact lithium-ion battery to power a USB port (and a 12v DC "cigarette lighter" socket). The M12 batteries have gobs of power. Street price on the power port is about $30, though it is a "bare tool", so you'll also need an M12 battery or two and the M12 charger.

The Zagg Sparq 2.0 is my absolute favorite. It has 2 USB ports and can charge up your iPhone/smartphone up to 100% 4x from dead. One of the USB ports is also optimized for the iPad and can charge it up to 60% from dead! You can even charge two devices at once! It is also a wall charger, so you can charge 2 USB devices and the Zaggsparq all at the same time.

I am an iPhone power user (with the unlimited plan :) and do a lot of traveling for business. When I am not getting business calls, I am on a flight listening to my music. Nothing is worse than having a dead phone in an airport with all your flight details in your email. It is great because you get a few days of charging out of it and can plug it in overnight with your device and everything is good to go. It is a little bulky, but it's not like your putting in your pocket, just throw it in your bag. I prefer to have the extra charging power and plug my phone in when I am not using it rather than carrying around a large case and extra battery weight in my pocket. $100 at Zagg.

If you're up for a project, consider a Minty Boost:

http://www.ladyada.net/make/mintyboost/

It works off of AA's, so you don't have to wait for it to charge itself. (I think Makershed.com offers kits as well.)

If you want a backup power source for your cell phone, the Goal0 12301 Nomad 7M Solar Panel is the best.

Not the most elegant, but for sure the cheapest: http://eshop.macsales.com/item/CellBoost/AC10371A/ It will set you back a whole $3. I think if you sign up for their spam you can get it for half that. I bought a few.

Designed for the original iPod shuffle, but works with any device that has a USB charging cord. Rechargeable from a USB port. It more than doubles the battery life of my iPhone.