Bonus for the curious

In case you're wondering exactly where and how WordPress stores the information about activated plugins, have a look in the database. Using your MySQL client, you can browse the wp_options table or execute the following query:

SELECT option_value FROM wp_options WHERE option_name='active_plugins';

The active plugins are stored as a serialized PHP hash, referencing the file containing the header. The following is an example of what the serialized hash might contain if you had activated a plugin named "Bad Example". You can use PHP's unserialize() function to parse the contents of this string into a PHP variable as in the following script:

<?php
   $active_plugin_str = 'a:1:{i:0;s:27:"bad-example/bad-example.php";}';
   print_r( unserialize($active_plugin_str) );
?>

And here's its output:

Array
(
    [0] => bad-example/bad-example.php
)