I've been thinking about interfaces to virtual versions of traditional musical instruments. I play the classical guitar, so I am particularly fascinated by guitar tuning.
The applet you're seeing would really only be exciting to play as an instrument if you use a multi-touch surface, such as the one developed by Jeff Han at NYU. But even with just a mouse and keyboard you can use this applet to learn your way around guitar tuning and chords.
This applet is also an exploration of synaesthesia: two independent senses perceiving the same phenomenon, since each musical pitch on the guitar is represented by a unique color - the 12 halftones in each octave form a rainbow spectrum, with higher notes being brighter.
Left-click on the strings to the left to vary the finger-chording You can also use the arrow keys to move around the finger-chording. Hold down the space bar when arrowing or clicking to move more than one string at a time (ie: to make bar chords).
You can also click on the preset chords or type the key combinations A, Dm, E7, etc.
To play the strings, either strum with your mouse across the strings at the right, hit the 1,2,3,4,5,6 keys to play one string, or the ENTER key to play all six strings.
Source: Guitar2 extends BufferedApplet, using synth/MidiSynth.