If you have a new iPhone, you have Siri... but if you're stuck with an original 4 or older and waiting for the 5 to come out like me, you should try Vlingo. I've been using it for a while and even put it to the test against Siri once, and for the most part, it worked just as well or better... and the voice even sounds basically the same.
Here a new spot that just came out to promote it and I just love it!