I've started playing with zombie.js recently as well - much lighter and faster than the ones that instrument a completely full browser. But has a full Javascript engine.
zombie.js is not a full browser. It's a poor emulation using jsdom as its backing. http://zombie.labnotes.org/guts Beware, for some applications, jsdom is super buggy.
We're also trying it for integration tests, as it is much quicker than Phantom or Selenium. Even there, where we control the standards-compliant site, it isn't quite good enough yet.
Would love to see more people helping make it so, though!