Here is a useful iPod Nano accessory, the Green Power GP 400N Sliding iPod Speaker.
The Green Power GP 400N is a case for your iPod Nano, that also features a slide out speaker, the speaker has a built in 600mAh rechargeable battery and a pair of 1.5w speakers.
The speakers will give you up to 12 hours of audio playback on one charge and your iPod Nano connects to the case via the dock connector, it also features a built in USB port so you can charge your iPod and the speaker with your PC or Mac.
We couldn’t find any pricing information on the manufacturers website, although it looks like the Green Power GP 400N is only available at the moment for the iPod Nano 4G.
Product Page via Craziest Gadgets