A piano smasher is a musical instrument invented by Blue Man Group. Piano Smasher is also the name of a song created by the same, featuring that instrument.
To create a piano smasher, strip a piano down to just its frame (with strings) and sound board. The strings are then all tuned to the same note; alternatively, each half of the piano smasher can be tuned to a different note for a two-tone instrument. The strings are then whacked with a really big mallet. Piano smashers of more than two notes are not used by Blue Man Group, as it becomes too easy to hit a wrong note.
A piano smasher produces a satisfying "bong" sort of sound. Because a piano smasher can only create one or two notes, songs that use the instrument generally use a plurality. A piano smasher is not usually the primary instrument at any point during a song; its nature makes it much more suited as harmony.
A piano smasher is not without its practical difficulties. According to the creators of Blue Man Group, on their message boards, a piano smasher used in a live concert is played with pre-recorded samples as the instrument tends to go out of tune extremely rapidly and more than a few microphones have been accidentally smashed with particularly unfortunate aim on a note. Triggers are placed behind the strings of the instrument, and the impact of the mallet activates them. The force with which the trigger is struck determines the volume at which the system plays the note. Also note that a piano smasher heard on any official recording that is not of a live show is an "authentic" Piano Smasher, and not altered in any way by a computer.
The song named Piano Smasher, heard on Blue Man Group's CD "The Complex", is the seventh track on the disc. Unlike many other tracks in The Complex, it has no lyrics and is a purely instrumental piece. As might be expected from a song based on an instrument like that, it is driven almost entirely by percussion. It is also a whole lot of fun to listen to, and one may find onesself accidentally dancing to it and looking like an idiot in the middle of a computer lab should one be listening to it on one's MP3 player. I have a great deal of experience with this phenomenon.