Example:
sudo ipfw pipe 1 config bw 15KByte/s
creates a pipe that only allows up to 15KB/s to go through.
Then:
sudo ipfw add 1 pipe 1 src-port 80
will attach that pipe to the outgoing traffic on port 80, effectively limiting the outgoing traffic of the web server.
sudo ipfw delete 1
will remove the pipe from the port.
[kirkmc adds: I haven't tested this. Just make sure you remember to turn this off when you no longer need it!]