Not so much a case as a portable mini-speaker, but still worth mentioning.
The Wireless Ground iPhone 4 Sound Amplifier acts as a mini ear-trumpet for your iPhone. You slot your phone into the Sound Amplifier, play music through the iPhone speakers and the Sound Amplifier makes it louder. Simple.
It’s definitely cheaper than buying a hi-tech speaker system, although music obviously won’t sound as good through the Sound Amplifier as it would through proper speakers. It also isn’t suitable for use at, say, a party or in a bar because it just doesn’t make the music loud enough. However, if you ever wanted to play music at a picnic or whilst you were in the bath, it’s perfect.
The only real downside is that it’s a bit unstable and you can’t use a case with it so you risk damaging your phone. But for what it is, the Wireless Ground Amplifier is excellent.