While some are serious business, Node.js for example, others are on the fun side of things. Dance.js is an example of the latter, as the name implies. It's described as a "high-level audio API, designed to make sweet visualizations" and it's available
on GitHub, natch.
It works with Mozilla's Audio Data API and the WebKit Web Audio API, but it can uses Flash too, if all else fails. There's a sweet and brostep-heavy demo
for it here. It's using HTML5 Canvas and WebGL for the visualization, so you may get some low frame-rates if you're going full screen.