OSX comes with ipfw (at /sbin/ipfw) which is an older tool much like iptables (on Linux) or pf (which I believe is more common than ipfw on BSD systems now). Since these tools are kernel-based and OSX's kernel is BSD-based rather than the Linux kernel, it makes sense that you find a BSD firewall tool rather than iptables.

http://en.wikipedia.org/wiki/Ipfirewall

http://en.roolz.org/Blog/Entries/2009/11/6_OpenBSD_PF_on_Mac...