Since our connection maxes out at about 160KB/s, I wanted to throttle my download bandwidth to 100KB/s. Here’s how you can accomplish this using ipfw.

1) Open Applications/Utilities/Terminal.
2) At the prompt, type:sudo ipfw pipe 1 config bw 100KByte/s
That will create a new “pipe” with a 100KB/s maximum throughput.
3) Type:sudo ipfw add 1 pipe 1 src-port 80
That will assign the pipe to all traffic on port 80. You can substitute whatever port you are using.

That's it! Your bandwidth on port 80 (including your download) will be throttled down to 100KBs. You can even do this during an existing download and it shouldn't break anything.

To delete the pipe:sudo ipfw delete 1
That will get you back to full speed. Or, you can just reboot your computer.

Note: tested on Mac OS X 10.5.8, but this should work on all versions of Mac OS X 10.4, 10.5 and 10.6 (at least).