Advanced OS X users know that Darwin comes with ipfw, which can be used to set up a custom firewall.
This same service however can be used to also limit bandwidth on specific ports.
For 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.
...and sudo ipfw delete 1 will remove the pipe from the port.
(I haven't tested this last bit. Just make sure you remember to turn this off when you no longer need it!).
