When I bought my new iPhone 3GS, it of course came with a USB power adapter. I already had one from my old iPhone, so this meant I had two of them. I put the new one in another room in my house. It is handy to have an extra charger to keep one in a different location in your house, in your office, in your summer home, in your yacht, etc.
You can buy a replacement USB Power Adapter from Apple for $29, but Art of the iPhone points out that you can currently get one from a vendor on Amazon for less than $7. Click here to get an Apple USB power adapter for only $6.91. Unlike the one from Apple, it appears that this one doesn't include the cord that goes from the USB adapter to your iPhone, but you can click here to get one of those cords from another vendor on Amazon for only $0.43. Note that for both of these sellers, you will also need to pay for shipping, even if you are an Amazon Prime customer. Thus, in addition to the $7.34 for the two products, you will need to pay another $9.88 for shipping for both. But at $17.22, you are still saving a little bit of money over the $29 (plus shipping) that Apple's online store charges.
By the way, for many of the items that I link to on Amazon, if you make a purchase after clicking on the link here, Amazon pays me an incredibly small percentage as a referral fee. You have likely seen this before on other websites. The same is true for items such as apps that I link to on iTunes, although since most iPhone apps are so cheap that doesn't amount to very much. It doesn't cost you any extra when you buy things from these links, and the referral fees to me are very small, but they do help to defray some of the costs for this website such as the $15 a month I pay to Six Apart (who runs TypePad) to host this website. So if you want to help support iPhone J.D., click on Amazon and iTunes links on iPhone J.D. to purchase products. And if you want to really help iPhone J.D. whether or not you own that yacht yet, feel free to use this link and buy a few dozen copies of this book.