Mac OSX Tuning
This page contains information on tuning Mac OSX hosts connected at speeds of 1Gbps or higher. Note that most of the tuning settings described here will actually decrease performance of hosts connected at rates of OC3 (155 Mbps) or less, such as home users on Cable/DSL connections.
Starting with OSX 10.5, Apple added "self tuning TCP", and increased the default maximum socket buffer to 4MB, a much more reasonable value. Unfortunately they also reduced the default win_scale_factor to 3, which prevents the TCP window from ever going above 512K. Be sure to set net.inet.tcp.win_scale_factor as shown below.
Apply these settings to /etc/sysctl.conf, and reboot:
# set this for hosts connected at 1GE or higher
# for 10G hosts it would be nice to increase this too, but
# 4G seems to be the limit for some OSX installations
More information can be found in this OSX Network Tuning Guide.