I have sourceguardian and its very nice. I got a special deal since I know the owner.
Fantastico also uses Sourceguardian for those that don't know. Ioncube is also very nice and you don't have to buy the full application. You can encode files online at their website for quite a reasonable price.
http://www.ioncube.com/pricing.php
The current encoder versions of ioncube, zend and sourceguardian are about the same security wise.