##In the terminal:
$ sudo ipfw pipe 1 config bw 15KByte/s
$ sudo ipfw add 1 pipe 1 src-port 80
This will restrict all incoming bandwidth to 15 Kb. Change it to whatever value you like.
E.g.
$ sudo ipfw pipe 1 config bw 2KByte/s
$ sudo ipfw add 1 pipe 1 src-port 80
To remove throttling:
$ sudo ipfw delete 1