If you don't mind getting your hands a little dirty, you can do this from the command line (open Terminal in Applications/Utilities). The simplest form would be this:
sudo ipfw pipe 1 config bw 300kbit/s
sudo ipfw add pipe 1 dst-ip 0.0.0.0/0
Enter your own password when prompted. Change the "300" to whatever number you want (for real fun, simulate a modem by changing it to about 30 or 40 kbit/s
).
This limits ALL traffic, including that to the local network (maybe you share files with other computers, and you don't want that to be slow). If you want to only limit internet traffic, there are a couple of options. One is to change the above rules to only apply to specific network ports - like web, mail, etc. To limit just those, do this instead:
sudo ipfw pipe 1 config bw 300kbit/s
sudo ipfw add pipe 1 dst-port http
sudo ipfw add pipe 1 dst-port https
sudo ipfw add pipe 1 dst-port pop3
sudo ipfw add pipe 1 dst-port pop3s
sudo ipfw add pipe 1 dst-port imap
sudo ipfw add pipe 1 dst-port imaps
Another option is to only limit traffic not going to or from the local network. If you know your network address (usually your internal IP with a 0 in the last place), you'd do something like this:
sudo ipfw pipe 1 config bw 300kbit/s
sudo ipfw add pipe 1 not src-ip 192.168.1.0/24
sudo ipfw add pipe 1 not dst-ip 192.168.1.0/24
(I used 192.168.1.0 as the network address; change it to yours).
Finally, the all important command: when you want to get rid of the bandwidth limiting, use this:
sudo ipfw flush
Also, rebooting will clear it - these settings don't stick between reboots.
More info is
here (it explains how to use queues to limit upload and download separately) or use
this Google search.
Hope this helps.
