Bikeometer: works great for tracking bike rides and hikes. Small, keeps track of mileage and speed, duration. Overlays your ride to google maps, so you can review exactly where you went. Unlike some apps that are 10x the size, you don't need to create an account or login with facebook or anything stupid like that.

Magnifier: uses your camera to zoom in really well, you can use the flash and take photos. I use it all the time looking at coins when I can't read a date or mint mark. Much easier to use than just the camera by itself.

Soundmeter: gives you a visual reference to how loud whatever's going on around you is. You get either a bar graph of dial needle, and it gives you benchmarks of common references such as normal conversation, street traffic, heavy machinery, etc. I like having this when I'm listening to live music. Lets me know when to leave, and actually got one band banned from returning (no need to play horn instruments into amplification when in a small indoor brewery. People left in doves).