A friend showed me a little device he had bought that impressed me so much that I bought one for myself. It's a small (and I mean very small) powered loudspeaker for use with iPods, etc, called the x-mini. It folds down to smaller than a tennis ball.
In use it unclips and expands to become a proper resonator.
Pictured here expanded along with the carry bag and recharging lead that come with it. You can also see the connecting lead that clips to the base when not in use.
To give an idea of scale, here's one connected to an iPod Touch, with a tape measure, a £1 coin and a 1 Euro coin.
In use the quality of sound is stunning! I would never have believed that you could get such a high quality and volume of sound out of something so small. I had expected it to sound really 'tinny', but it manages lower frequencies very well.
It has a 3.5mm output socket, so that you can 'daisy chain' another x-mini for more volume. This socket is useful if you want to use it with a radio or an Ipod with a radio function.
Personal radios use the earphone lead as an antenna, so the short lead of the x-mini doesn't work very well. If you plug another lead into the x-mini's output, it will act as an antenna and pick up radio stations adequately.
The battery is recharged from any USB port via the supplied lead, so can be plugged into a laptop computer.
Ideal for carrying on a bike when touring as it allows you to listen to your Ipod or radio in your hotel room or tent. If you don't have a laptop with you, you can always recharge it from the USB port you can fit to your bike (next 'Useful things' post).
I got mine from Amazon, where strangely it's cheaper in black. There are loads of other people selling them on the Web, so hunt around for the best price.