In case it hasn't been mentioned here yet, for those with the Google Now / Google Search app, you can now set it to respond to spoken 'OK Google' from any screen (even, optionally, the lock screen!).
Go to Google Search -> Menu -> Settings -> Voice -> Language and change to English (US)
Under Google Search -> Menu -> Settings -> Voice -> “Ok Google” Detection -> Enable From Any Screen.
If the 'Enable From Any Screen' option isn't visible, I've heard it can be made visible by doing search for "Okay Google Everywhere" in Google search.
You'll also see an option to enable it from the lock screen, but bear in mind that enabling “Ok Google” detection from the lock screen will allow navigation commands, email and text commands, etc. to be executed without unlocking, which might be seen as a security issue.
