How can I tweak Macintosh OS X ?

The tweaks apply to TCP/IP, and the protocol is OS independent. There are a couple of GUI programs floating around the net for changing the values under Mac OS X that you might want to find, or below is a quick manual fix. First, start Terminal.app and sudo (login as root user). You either have to type the following each time you reboot, or put it in some sort of startup script:

/usr/sbin/sysctl -w net.inet.tcp.sendspace=256960
/usr/sbin/sysctl -w net.inet.tcp.recvspace=256960
/usr/sbin/sysctl -w net.inet.udp.recvspace=65535
/usr/sbin/sysctl -w net.inet.tcp.rfc1323=1
/usr/sbin/sysctl -w net.inet.tcp.delayed_ack=1
ifconfig en0 mtu 1500
ifconfig en1 mtu 1500

Notes: In the first two lines, change the Rcv/Send Window as needed. In the last two lines, you can just use the one that's your Internet connection, and change MTU as needed.

Below is a list of some related GUI applications for Macintosh OS X:

Carrafix.com
Download Accelerator Plus
Throttled
SpeedDownload 2

If you've found other reliable GUI applications for Macintosh tweaking, please let us know so we can add to this FAQ.