There are those who buy the iPhone for silly games, and then there are those who buy the iPhone for manly purposes like measuring tables and making sure shelves are made level. While you were putzing around playing Doodle Jump and Pocket God, these people were flicking their digital Zippo lighters in peoples faces, just because they could. They chugged beer after beer whenever anyone walked by and whipped everyone who wasn't working at their Mountain Dew pace.
These toolbox apps for for the tough guys out there, and maybe for a few of the guys like me who have to hang a picture every once in a while. Here's to you, tough guys.
Ruler 2 - Sure, the average guy that needs a measurement probably has a ruler of some sort laying around, but if you don't, save a tree and buy Ruler 2. With Ruler 2 you can measure an object 2 15/16th of an inch at a time, as long as you make sure to mark where the end of the screen touches. With it, you can measure the smallest of objects, or your whole car if you have a good deal of time.
The app also instantly converts inches to centimeters with a touch, so you'll never have to try to do the crazy conversion in your head. Nobody wants to have to multiply things my 2.54... it's just too much work.
iHandy Carpenter/ iHandy Level - Unlike the ruler, there just aren't that many people out there that have levels. The master of all tools, the level makes sure that all of your pictures are hung straight on the wall. All you have to do with the app is calibrate it on something that is level, and then just hold it up. The app will then detect any angle at which the phone might be at with a visual level bubble, just like if you were using the real thing.
If you want to get really handy, you can move on from the free level to the iHandy Carpenter. This app has a level, a surface level, a protractor, a ruler, and a plumb bob. Just make sure that you have a big budget for supplies because you may want to go on and build a house.
Flashlight + - Probably the most consistently useful app on the list, Flashlight + uses the LED flash on the iPhone camera to act as a flashlight. It's just as bright as any flashlight you might own, and it never needs AAA batteries.
The competition in the App Store is steep for flashlight apps, but Flashlight + has a UI that is as slick as any app that is built into your phone. Just for kicks, it also has a strobe light that will work great for Halloween.
DAH-Measure - While most of the objects that you will have to measure will be small enough to use the either of the ruler apps, DAH-Measure is the only one that is handy enough to measure something like a tree. Using the iPhone's camera as a guide, DAH-Measure can roughly measure the length or height of something with a few fancy equations.
Just a warning, because the app does everything by sight, it's not always on the money with the exact measurement. You'll be surprised though how close it actually gets.