Google Now is arguably the most interesting voice command application out there but only a limited number of Android devices have support for the service right now. However Google Now's market will increase greatly in the following months with the release of Google Now for iOS.
A leaked video shows an official promo for the service designed for iPhones and iPads. The app will be integrated into Google's Search app already available on iOS. The service will work much as it does on the Android platform. It aggregates different types of data such as location and time and shows you cards with important information that you might need: such as traffic jams on your way home, or weather alerts, or flight delays. Google Now will offer the same, very robust, audio recognition service so you can interact with it using only your voice.
It's not clear when the service will launch, but expect to be quite soon as Google seem poised to greatly expand this service.