Auditorium is perhaps the most interesting browser game I’ve ever played.

A stream of white particles flows across the screen, and by strategically placing controls in its path you can redirect it towards audio “audio containers” that make music as the particles flow across. The result is a beautiful song that sounds just right only when the stream is properly flowing through each container.

Auditorium

The full game costs $10, but you can try several “acts” for free (without any form of registration or account required — you just visit the website).