Also, according to the MMCache homepage, MMCache is only outperformed by the Zend Accelerator suite.
MMCache also has an encoder (like PHP Accelerator) but it's totally free as well. Great for if you want to encode your PHP and distribute it without source.
Either way, if you're running a busy PostNuke site, you should be running one of these PHP Caching modules to get the maximum speed from your website. Performance gains are around the 450% mark with MMCache. Not bad for a 90k Zend extension!