The lifeblood of your iOS device is its battery, and you want to make sure to keep it charged. All iOS devices ship with a USB power adapter, but a variety of third-party solutions are also available. It doesn't hurt to have an extra one on hand (i.e., one for home and one for the office). You can also get a cigarette lighter adapter to power/charge your device while you commute. Ultra portable solutions include solar chargers and rechargeable battery packs in a variety of styles. A good battery pack can double or even triple your battery life.
Satechi iCell 4800 mAh Battery Extender Pack and Charger ($59.99, satechi.net) is one of the smallest battery extenders and can fully recharge your battery at least three times.
Griffin PowerJolt Reserve ($39.99, griffintechnology.com) You can extend your surfing, talking, and tune time with the PowerJolt Reserve. It is not yet compatible with iPhone 4.
Kensington Mini Battery Extender ($39.99, kensington.com) extends playing time for your device up to 15 hours of music, 4 hours of video, and 3 hours of talk.
Morphie Juice Pack Air ($79.95, mophie.com) This form-fitting case has a battery extender concealed inside. You get hard-shell protection plus an additional 4.5 hours of talk time or 27 hours of audio playback for your 3G and 3GS.
Case-Mate Fuel ($49.99, case-mate.com) provides a protective case, a carrying solution, and an extended battery life all in one. It is not yet available for the iPhone 4.
NOVOTHINK's Surge ($79.95, novothink.com) With this hybrid solar charger case, you can now power-up your iPhone literally anywhere under the sun. It is available for iPhone 3G and 3GS and coming soon (hopefully by the time you're reading this) to iPhone 4.