With the many new features brought to us with iOS 8, "Hey Siri" allows the user to say exactly that and Siri will activate by voice. However the issue(?) here is that this only works when the iOS device is plugged into the charger. The question mark found above is because I'm wondering if we really want Siri to activate accidentally in our pockets when it hears something remotely similar.
UntetheredHeySiri is a Jailbreak Tweak that takes away the limitations on the Hey Siri feature. After installing the Tweak, the user can choose in Settings to have Hey Siri activate only when plugged in, or activate anywhere, even when not on a charger.
I have a bit of a love hate relationship with this Tweak, sometime I'll have my iPhone 6 Plus plugged in at home, I'll be watching a movie and Siri will activate because it thought it heard "Hey Siri". Either way, UntetheredHeySiri is a free Tweak and now available in the Cydia Store.