Google's latest version of its voice-enabled Search app may not have the personality of Siri, but it is as good, if not better, than Siri at pulling up relevant search information. If you want to replace Apple's personal assistant with Google's version, you can do so thanks to a jailbreak tweak called NowNow.
As spotted by AppAdvice, NowNow is available in Cydia and allows you to launch Google Voice Search anywhere in iOS. It requires you to install Activator and Google Voice Search, but once you set it up, it's as easy to use as Siri. If you drop Siri for Google, remember that you will lose the "assistant" features of Siri, like adding reminders and setting alarms.
Your device has to be jailbroken to use this tweak, which means it is not available to iPhone 5 users or anyone with an iOS device with an A5/6 processor on iOS 6.