This is a very simple bash script to throttle your bandwidth for testing purposes. Right after writing this I discovered that someone else also wrote something similar: https://gist.github.com/j-manu/1409218

throttle.sh

#/bin/bash

THROTTLE=200
PORT="any"
LINE="--------------------------------------------------------------------------------"
start()
{
    echo "Throttling data from port $PORT to $THROTTLE KByte/s"
    
    unit="KBytes/s"
    throttleString=$THROTTLE$unit
    sudo ipfw pipe 1 config bw $throttleString
    sudo ipfw add 1 pipe 1 src-port ${PORT}
}

stop()
{
    echo "Stopping throttle."
    sudo ipfw delete 1
}

status()
{
    pipe=`sudo ipfw list | grep 'pipe 1'`
    if [ "$pipe" ] ; then
        echo "Status: Active."
        echo "$LINE"
        echo "$pipe"
        echo "$LINE"
        echo ""
        echo "Pipe details: (Hold onto your butts)"
        echo "$LINE"
        sudo ipfw pipe 1 show
        echo "$LINE"
    else
        echo "Status: Not Active."
    fi
}

# check if 2nd arg is a number and save as throttle speed
if [ "$2" ] ; then
    if ! [[ "$2" =~ ^[0-9]+$ ]] ; then
        exec >&2; echo "Error: Throttle speed is not a number"; exit 1
    fi
    THROTTLE=$2
fi

# check if #3 arg is either 'any' or a port number
if [ "$3" ] ; then
    if [ "$3" != "any" ] ; then
        if ! [[ "$3" =~ ^[0-9]+$ ]] ; then
            exec >&2; echo "Error: PORT is not a number"; exit 1
        fi
    fi
    PORT=$3
fi

case "$1" in
    start)
        start
        ;;
     
    stop)
        stop
        ;;
     
    status)
        status
        ;;
    *)
        echo $"Usage: $0 {start|stop|status} <speed in KB> <port number>"
        exit 1
esac