Custom Launcher that supports Google Now integration?
The Google Experience Launcher is actually built into the Google Search app along with Google Now. This is why it can listen for 'Okay Google' and have Google Now accessible via the left swipe. Ars Technica has a good article which goes into more details on this.

There are some launchers that support gestures, which might be similar to the left swipe to access the Google Now page (although they will only be able to launch Google Now in the same way an upward swipe does).

I don't think there are any that support saying 'Okay Google' from the homescreen though.