Ok! Interesting. Are web workers gaining traction? This is the first time I've heard of them. Are there any major users/libraries that depend on ww?

It depends on your browser support requirements. On the Brackets project[1], we work within Chromium (and "modern browsers" with our in-browser branch), so we can rely on web workers for doing things like parsing JavaScript for autocompletion.

Web workers are quite useful and I'm sure there are a fair number of webapps that take advantage of them.

[1]: http://brackets.io/