MP3Trigger by David Wicks
http://sansumbrella.com/

Simple wrapper for MP3Trigger serial communication from Arduino.
Also allows you to loop tracks.

You need to place MP3Trigger in the right place on your machine
for Arduino libraries. On a Mac, that means MP3Trigger.h and
MP3Trigger.cpp live inside:
~/Documents/Arduino/libraries/MP3Trigger

For sample usage, see the examples/ folder (courtesy Carl Jensen)

Thanks to:
Carl Jensen, http://dimsos.fridata.dk


Sparkfun product page has link to User Guide which they call
Datasheet for some reason: 
http://www.sparkfun.com/products/9715

Updates on new firmware for MP3 Trigger V2 at:
http://makerjam.com/support/