NowNow lets jailbroken iPhones use Google Voice Search instead of Siri
Of course, this does come with its own limitations. Google is obviously not allowed to hook into the hardware and software in iOS like Siri, so you won't have access to setting reminders, sending messages, etc. (unlike alternative voice command options on Android). NowNow is just going to give you easy access to Google Voice Search, but, if you're on an older iOS device that doesn't have Siri, that's still an upgrade.
You need to have a jailbroken iOS device in order to install NowNow, because it's found in the Cydia App Store. Another thing to be wary of here is that NowNow requires you also install another Cydia app called Activator, which has a troubling reputation as a resource hog. Those issues aside, once you install NowNow, you'll be able to map Google Voice Search to a number of shortcuts including various interactions (long press or multi-click) with the Home button.