This is undoubtedly one of the coolest free apps you can download. Just launch Sky Map, and point your phone to the heavens (or any direction at all, really). For the most mind-blowing results, you'll want to be outside at night, but it'll work during the day, too. You'll see a map of the constellations, planets, and Messier objects that currently occupy the portion of space at which your phone is currently pointing. The app will even point out meteor showers while they're happening.
(Did we mention it's Android-only? IPhone has a few less interesting and more expensive versions.)
Oh, and it does time-travel, too. Star Maps can show you what the sky will look like in the future, or what it looked like in the past, as far back as 1900. Ever wonder what the sky looked like the night of the Apollo 11 moon landing? Now you can find out, in more detail than you probably wanted to know.