Phonophone III: If you want to get fancy, this Phonophone III for the iPhone is one of the fanciest iPhone amps I've seen. It's made from ceramic and has a dock for the iPhone to sit in inside the speaker. It amplifies the sound to 60 DB and will definitely make a statement sitting on your desk or at home.
Arkanary II: The Arkanary II brings a little old school design to your modern iPhone. The Arkanary comes with a clip that attaches to the bottom of the iPhone where the speakers are and amplifies the sound naturally through its large speaker shape. There's something about the classic design of it that I'm drawn to and I've got my eye on the gold one.
Griffin Air Curve: One of the popular favorites, the Griffin Air Curve amplifies the sound of your iPhone without any batteries, cables or cords. Basically the amplifier was designed to complement the speakers on the iPhone, increasing the sound to 10 DB louder. I also like the modern looking clear design.
EcoAmp: If you want to enjoy better listening on your phone but decrease your ecological footprint, EcoAmp is a non-electronic solution. It's made from recycled paper and the handy instruction video tells you how to fold the paper and turn it into a functioning speaker. The set comes with two.
Soundbrix: Another eco-friendly solution, is a dock designed to look like building blocks. Soundbrix works like any portable speaker, you place your phone in the dock and the sound is immediately increased by 15 DB without any power. The way the dock is built amplifies the sound naturally. The best part is it comes in different fun colors.
(Images: as linked above)