UPDATE: if Internet Sharing is enabled, the rate limiting described in this page does not work. Turn it off to have rate limiting.
UPDATE 2: There is the option of using Network Link Conditioner, a tool for developers that adds a pane to System Preferences. I guess you need a developer account to download it. This tool has ready-to-use profiles to simulate common network technologies like 3G, EDGE, DSL etc.
Just a quick script: a very simple bandwidth limiter for OSX.
#!/bin/bash -x ipfw flush ipfw pipe 666 config bw 4000kbit/s ipfw pipe 667 config bw 600kbit/s ipfw add pipe 666 src-port 443 ipfw add pipe 667 dst-port 443
The example above limits download from port 443 (http) at 4MiB/s and upload to the same port at 0.6MiB/s.
I use this to avoid monopolizing a 10/1Mbps cable connection while uploading videos to YouTube. Such services should include a bandwidth limiter, but anyway.