Google Sky Map
A virtual planetarium at your fingertips, Google Sky Map may be the coolest Android app in the market. Using GPS as well as the Android phone's internal accelerometer and compass, the app determines your position and displays a live map of the planets, stars, and constellations on the screen. Simply hold the phone as if you were taking a picture of the heavens, and Sky Map will display on the screen that portion of the sky directly behind your phone.
Can't figure out the name of a constellation? Point the back of the phone at it, and Sky Map will tell you. If you've ever gazed up and wondered whether a body is a star or a planet, Sky Map will identify it, as easily as if you were carrying a little stellar cartography department in your pocket.
A free application, Sky Map is a must-have, and one of the few Android apps that will make even the snobbiest iPhone user's jaw drop.
The app does occasionally become confused when it can't tell if the phone is horizontally or vertically oriented (such as when you're pointing the handset straight up at the zenith of the sky). In addition, though Sky Map draws the lines between major stars in constellations, I noticed a few cases (particularly in the constellation Cassiopeia) where the lines appearing in Sky Map didn't match up with my own understanding of the constellation or with any other star chart I've looked at. But in the big scheme of things, does it really matter? I simply turned off the constellation lines on my phone and marveled at the universe through a 3-inch touchscreen.