<p>I have two controllers which share most of their code (but must be, nonetheless, different controllers). The obvious solution (to me, at least) is to create a class, and make the two controllers inherit from it. The thing is... where to put it? Now I have it in app_controller.php, but it's kind of messy there.</p>

In cake, components are used to store logic that can be used by multiple controllers.  The directory is /app/controllers/components.  For instance, if you had some sharable utility logic, you would have an object called UtilComponent and a file in /app/controlers/components called UtilComponent.php.
```
```
<?php
class UtilComponent extends Object {
    function yourMethod($param) {
        // logic here.......

        return $param;
    }
}
?>
```
```
Then, in your controller classes, you would add:
```
```
var $components = array('Util');
```
```
Then you call the methods like:
```
```
$this->Util->yourMethod($yourparam);
```
```
More Info:
Documentation