We have seen a few of these speakers before on Geeky Gadgets, that turn any surface into a speaker, the latest one is the Tunebug Vibe and it looks like is it one of the coolest so far.
The Tunebug Vibe measures 2.5 inches in diameter, and it can convert any surface into a speaker just by placing it on it.
Depending on what surface you place it on, the sound will be slightly different, so you could have some fun trying different surfaces to work out which is the best sound.
It works with the majority of MP3 players and any gadget with a 3.5mm audio jack, and it can be recharged via USB.
If you want one, the Tunebug Vibe is available for $69.95 from Tunebug.