not an essential app, but they have a cool called gun disassembly 2, Allows you to see 3d models of many guns being taken apart, and also put back together. It also has a game part where you can see how fast you can breakdown the guns.
Also weatherbug app is pretty useful for weather alerts and radar.
There are so many fun and/or useful apps for the android platform, I never know where to start when asked this question. Let's see, there's: Flashlight, compass, survival guides, guitar tuner, metal detector, vibrometer, file sharing (torrent pirating), FM radio, brain training games, word games, zombie and fishing and racing games, on and on. You should definitely check out Google Goggles, an app that takes a picture, scans it, then google searches the image for you. A barcode scanner is a really useful consumer tool. I also recommend a backup app. I use Mobile Backup II. I use a night vision app that works surprisingly well called Night Vision Camera. There are also apps that can turn your phone into a security camera or a motion activated alarm. And the best part is that I have never paid for a single app on my phone. This list should get you started, but I still have a lot more to discover so keep looking and share if you find anything good.