In the lovely-and-cool category, I offer up the "Hammond Flower", an online interactive music-toy by Amit Pitaru. It's Amit's version of the Hammond B3 organ, one of his favorite instruments. As he describes it:

I've always enjoyed playing the B3 'backwards' - instead of first setting the drawbars and then playing the keyboard, i would hold one note down on the keyboard and play with the drawbars, investigating the ingredients of a note rather than the notes themselves. The Hammond Flower instrument lets me do just that. It provides an alternative interface to the B3's drawbars, and then extends the metaphor by adding a dissonance component. In addition, A visualization engine (center part) translates the overall sound texture into a visual architectural element.

It's totally mesmerizing.

(Thanks to Waxy.org for this one!)