At the moment, there's no cron job service supports cron jobs with interval less than 1 minute. If you're really in need of cron jobs every X seconds and your hosting supports PHP, you can use my cron.php (http://tinypaste.com/7bcc5).
First, edit the $cron_url, $time_interval, $real_time_interval. Then upload cron.php to your web root, i.e. can be accessed at http://yoursite.com/cron.php
Set cron job with URL to call http://yoursite.com/cron.php to run every minute.
Hope it helps :)