Mashbox was a somewhat ill-conceived "mashup toy" that was designed and built by a mobile development firm in Austin, TX. While it was fun to play with, it remains my biggest product flop to this day.
I worked with several of the other musicians on staff at Beatport to devise a layout that allowed a user to load the individual audio parts of two separate songs into the player at the same time. The audio parts were segmented by their purpose in the song (e.g., bassline, melody, etc). Individual parts from one song could be "triggered" along with parts from the other song, making it really simple to create a unique mashup of popular electronic music tracks.
I provided layout and behavior direction to the development firm, and they executed the visual design.
Here's a cute promo video for the Mashbox app that was made by Mutual Mobile (the mobile dev firm that build the app). :)