Star Walk is one of those applications that once you have it, you show it off to everyone you know. On my iPhone 3GS, I just launch the app and raise my phone up to the sky, positioning it like I'm looking through the phone. Star Walk automatically adjusts the view on the screen to show you annotated star charts of what you are looking out. So if you see a bright star in the sky, you can just raise you phone up to it and see what it's name is. Also great for finding constellations and searching for the position of stars.
But Star Walk is a lot more than that. They also include a time machine so that you can see what the stars look like at any time from any location. Astronomy picture of the day, integrated help, moon phases, and links to Wikipedia for more information on many of the objects in the app.