A French cleat is a rock solid way of hanging cabinets. I like for its ease of installation. One man can hang a cabinet on the cleat in seconds. It's almost like hanging your coat on a coat hanger. The cabinet will stay in place without screws and can be squared up without stress.
How it works
The cleat is nothing more than a piece of wood running horizontally along the upper part of the cabinet. The lower part of cleat beveled. Another piece of wood with the bevel going the other way is installed on the wall. When the two pieces meet on the wall, gravity will pull the pieces together. The cabinet hangs on the cleat. To keep the cabinet from moving, It is permanently attached to the wall with screws.
How to build a french cleat
There are no rules for building the cleat except that the bevel should be 20 degrees or more. The cleat can run the entire length of the cabinet, or be shorter. Multiple cleats can be used as well.
Other than that, make sure two parts of the cleat are attached to the cabinet and the wall in way that can handle the weight.
The main drawback of the cleat is the extra material needed and the time to install it on the cabinet. You won't find French cleats on commercial cabinets. The wood would be too expensive.
Commercial French Cleat
If you don't want to build the cleat out of wood, commercial versions made from metal are available in various lengths.The metal version takes up less space behind the cabinet, and saves time during the build.
For the hobby carpenter the French cleat the way to go. It supports the entire weight of the cabinet during installation. Nothing else will make cabinet installation this easy.