We don't all have the latest iPhone with the kewl Siri voice command software. I myself have a Droid.
However, we can still get software that comes close to doing what Siri does.
Google Voice Actions (Free) Doesn't understand language but, can be used to start calls, create and send email, get directions and do a Google search.
Pannous Voice Actions (Free on Android) Has a lot of Siri's functions. Allows voice commands, you can set appointments and reminders, make calls, and text.
Just use your phones Market application to find these applications.