Iris is a cross-platform and free app that was designed to be your pet application that does things similar to Siri.
For now, some of the features like play itunes, open email, etc are only available on Mac.
Most of the features such as use of VOICE or TEXT as feedback mechanism, as well as feedback message are configurable through the configuration file ''user_scripts/iris.config".
You can also modify any of the predefined apple scripts (for eg: playSong.scpt) in that folder which will be called by Iris on the voice cammand.
Also, for advance users, there are ten function commands that he/she can use to perform user-specific tasks.
Detailed instructions on how to install and use the Iris utility on your Mac are available HERE.
Iris is a Java written application and it can run on most OS, including Mac, Windows and Linux.