Jaymi previously has shown The Four Best Electricity-Free iPhone Speakers, but here is another beauty, the MegaPhone from Italian designers en&is. They all work on the same principle, as explained in Wikipedia:
The horn itself is a passive component and does not amplify the sound from the driving element as such, but rather improves the coupling efficiency between the speaker driver and the air....Acoustic horns convert large pressure variations with a small displacement into a low pressure variation with a large displacement.
This one is a beauty, handmade in Italy. The designers write:
Passive Amplifier for iphone made of ceramic. the form is designed to amplify and optimize the best sound output. the amplifier is based on a thin wooden frame that allows the object to float off the table. this in order to increase the vibration of the object and to optimize the emission of sound. designed for the iPhone is perfect for listening music without headphones, for audio conference to hear the person on the phone as live voice.
Sounds pretty good, although I think their choice of music might have been better. 399,00 € at en&is