We're all for multifunctionality, and the Kensington PowerLift (US$49.99) would seem to have that in spades, melding an external 1200mAh battery, stand, and docking station in one pocket-sized package. The PowerLift is compact enough to allow standard phone usage when attached, and the triangular structure supports your iPhone in both portrait and landscape viewing modes. Another plus: its extended-docking-plug design means that you probably won't have to remove your iPhone from its case before plugging it in.
Obviously, the biggest tradeoff for the compact size is battery life: 1200 mAh is on the low side of external batteries we've tested. Kensington advertises that an iPhone 4 plugged into the PowerLift can get up to 20 additional hours of music, 5 hours of video, 3.5 hours of talk, or 1.5 hours of FaceTime (your mileage may vary). Nonetheless, if the thought of sticking a bricklike external battery in your pants pocket doesn't thrill you, the 2.5" square, under 2 ounce PowerLift might be your "just right" solution.
If you're an iPhone 4 owner, and protection is more your thing, another US$10.00 will set you up with Kensington's PowerGuard battery case. It packs the same 1200mAh battery as the PowerLift, but opts for a hardshell case with integrated volume control and a MicroUSB connector for charging/syncing. The PowerGuard also doubles as a stand: a simple slit in the case back lets you use any standard credit card (or the case's own instruction card) as a kickstand for landscape viewing.