The Hang and Level is a pretty great tool for hanging pictures, mirrors and the like.
Every piece of art I hung in my near-an-active-fault-line home would go crooked in days, or hours. I got a Hang and Level, and now most of them stay in place. With built in levels, and easy press tabs to mark the wall, it is hard to go wrong -- but I still can.
Hang and Level picture hanging tool via Amazon
