Android: You can already use Google Now to do lots of cool things with just your voice, but it doesn't have everything. Commandr not only adds several new voice commands, it also allows you to create your own with Tasker.
The app works by utilising the "note to self" command. As veteran users know, you can set a default app with this command to choose your note-taking service. Commandr takes the input from the note to self command and turns it into custom voice commands. It may be a little awkward to say "note to self turn on GPS", but it can be a life saver if you're using your phone hands-free in the car.
Additionally, Commandr allows you to plugin to custom Tasker actions. We've talked about how to create custom Tasker voice actions before, but they require your phone to be constantly listening (which can be a battery drain). With Commandr, you can activate commands via Google Now, which is accessible in several ways, so it's always on hand.
The developer has also promised to add new voice commands and is accepting votes from users on which ones to develop first. It's a rare treat to see a developer seek such explicit input from users, so if you'd like to see something added, be sure to ask!