The free app will let Glass users configure their new face-computers, beam navigation prompts from phone to face, and screencast Glass's view onto an iPhone or iPad. Of course, the app is completely useless if you haven't got Glass; Google even says so in the description. But don't let that stop you from downloading it and pretending you're a Glass Explorer.
Google seems to have pulled the iOS app, at least for U.S. customers. Thanks KidBobot for the tip!
2nd Update: It's back, and you can download it here