Little Snitch OS X Firewall
Little Snitch is a host-based firewall for OS X that allows granular control of both in and outbound network connections. It has an intuitive and easy to use interface that pops up notifications when any application attempts to connect out to the Internet. Setting rules to temporarily or permanently allow or deny connections can be done with a single click. Little Snitch is a closed-source application that costs $29.95 but the demo version can be downloaded and ran for free. One completely free firewall alternative to Little Snitch called GlowWorm (version 1.5.2) is not recommended because it causes OS X 10.5.4 to crash upon rebooting and requires loading in to single user mode to disable the application.