Virtual assistants that can get things done and can understand natural speech are not limited to the iPhone 4S, however: There is a solid selection of third-party apps for both Android and iOS that can accomplish much of what Siri can (though obviously not with the same level of ground-floor integration into the system as Siri).
Vlingo (Android and iOS, Free)
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"Looking for Siri-like functionality on your Android device?" asks the product description for Vlingo
in the Android Market. "Look no further."
Vlingo, actually available on both Android and iOS, does not understand natural language like Siri does; the list of accepted commands that you can use with Vlingo appears on the app's home screen. Vlingo still has a good amount of functionality, included some actions that Siri still cannot do: It can (again, per the Android Market description) send texts and emails, voice dial, search the web, find local businesses, get directions, update Facebook and Twitter, check in to Foursquare and launch apps. It has also added a very cool (beta) InCar edition for totally hands-free voice command when you're driving. Neat stuff.
Speaking of neat, Vlingo can also be chosen as the default Voice Input within the settings of your Android phone for direct access when touching the microphone key on the keyboard.