Voice recognition is one of the best ways to control Arduino projects, but it's not exactly easy to implement on your own. Instructables user ASCAS shows off how to add voice control to your Arduino projects using your Android phone and Bluetooth.
The Arduino gets a Bluetooth attachment and a set of code. Then you'll need Android Meets Robots, an app that lets you pair voice recognition with any Bluetooth module. From there, it's just about pairing the two together and creating your own commands. It's a pretty simple set up process that could potentially control a ton of projects. Head over to Instructables for the full guide.
Voice Activated Arduino [Instructables]