OS X includes a basic firewall that helps to protect your Mac from a variety of outside intruders, such as hackers trying to run spam-spewing robots. In Leopard, though, the firewall-which is disabled by default-offers very little in the way of customization. At most you can specify programs and services (such as file sharing and screen sharing) to which outside connections are explicitly permitted or blocked. IPNetSentryX stands at the opposite end of the firewall software spectrum. The program from Sustainable Softworks has every bell and whistle that even the geekiest Mac user could hope for-and a complex user interface to match.