The Standard PHP Library (SPL) has quite a few exception classes that are useful right out of the box.  Additionally, these exceptions can be extended just like the Exception and ErrorException classes.

My favorite is the InvalidArgumentException.  I use this exception if some parameter to a function or method is not what I expected.

<?php
 
function doubleMe($number)
{
    if (!is_numeric($number)) {
        throw new InvalidArgumentException(
            'Unable to double a non-numeric value');
    }
}

If you are using the __call() magic method, the BadMethodCallException is another great built-in exception to use.

class Foo
{
    public function __call($name, $params)
    {
        if (!method_exists($this, $name)) {
            throw new BadMethodCallException(
                "Method $name does not exist");
        }
    }
}

The SPL exception classes are a great alternative to writing your own exceptions classes. For some more examples you can check out PHPUnit. Sebastian Bergmann uses a lot of SPL exceptions in his code there.