Here's a great little tip - something I only discovered the other day and that really made me smile.  For a long time I've been using a great little app called iHandy Level.  It uses the motion sensor inside the iPhone to give you an approximate idea if something is level or not.  I don't use it a lot, but a few times I've been caught in a situation where I needed to roughly level something and had no level to hand.

levelNow, in iOS7, the Level feature is fully incorporated, but in a slightly less than obvious place.  Do a little search on your iPhone for Compass.  Open the compass app and calibrate it as suggested.  Nice compass right?  Now, slide to the left with your finger and voila!  There's the Level app built right in.  Try it out.... pop it on your desk and see if it's level.  When it hits 0 degrees, the screen turns green to indicate it's level.  I wouldn't rely on it 100%, but it's neat little feature eh?!?