Vlingo - Vlingo doesn't just take voice commands to make calls, SMS, emails, search the web, or get directions, it does something Siri can't - interfaces with your Twitter and Facebook accounts to let you update with your voice . . . and it really works, even without workarounds.
Dragon Go - Available for iPhone and iPod Touch owners running iOS 4.0 or later, this app works similarly to Siri in that it will interpret and take commands from your voice. You can ask for directions, to draft an email or SMS, and get show times to upcoming movies in your area by directing you to external websites.
Google - Google Mobile's voice abilities let you find information available on Search. For example, you can search with your voice for movie times, photos, websites, nearby restaurants, and more.
Voice Control - Your iPhone's included voice search isn't as powerful as Siri (or any of these other apps), but it does the trick when making calls, searching music on your phone, and getting the time. It's a good start, anyway.