This portable phone speaker uses a LM380 amplifier chip and some other basic components. It's not hard to build and the result is a powerful and practical custom speaker that's compatible with any device that outputs audio via a 3.5mm jack.
Written tutorial - http://x2jiggy.com/blog/2012/12/31/portable-phone-speaker.html
Amplifier schematic - http://electronicszone.net46.net/circuits/simple-25w-lm380-amplifier
LM380 datasheet - http://www.ti.com/lit/ds/symlink/lm380.pdf