IPNetSentryX

IPNetSentry is a simple and intelligent security application which protects your Mac from outside Internet intruders.

This is particularly important for Mac users who have cable modem, DSL, or another high-speed Internet service where connections can be maintained and left unattended for hours (or days) at a time.
Unlike most other Internet security products, IPNetSentry does not erect barriers for the safe use of your Internet connection. There is no need to "punch holes" in a firewall for specific applications you may wish to run. Instead, IPNetSentry silently and intelligently watches for suspicious behavior, and when triggered, invokes a solid filter which completely bans the potential intruder from your Mac.