Looking for multi input single output to headphone device

I'm looking for a device that would allow me to connect several input devices to it and a headphone and be able to hear the output from all of the devices. Specifically I want something to put on my motorcycle so that I can listen to my MP3 player through headphones but still be able to hear my radar detector when it goes off. I found one such device at http://www.electric-avenues.com/amplirider.html but I want to explore all my options.

It's called a mixer, you can have either variable or preset gains on the inputs - a single opamp and a few resistors is probably the easiest way to do it!.