I have bash aliases for various levels of ipfw throttling, but I've ended up using other programs because 768k/s with no latency seems to load some kinds of sites disturbingly close to the speed they load if I let the bandwidth back up to its 18mbit/s normal speed, which I think has to do with latency. Lots of the Mac OS standalone programs that do this kind of thing, like Link Conditioner or Speed Limit, add configurable latency to requests as well.
The reason Link Conditioner is 10.7 and up is that it's using pf[0] internally. The man page for pfctl[1] is pretty easy to read after you start to get the basics of how pf works. Be careful applying examples you find online though. Last I heard, OS X's version of pf is slightly newer that FreeBSDs, and moderately behind OpenBSDs.
I have bash aliases for various levels of ipfw throttling, but I've ended up using other programs because 768k/s with no latency seems to load some kinds of sites disturbingly close to the speed they load if I let the bandwidth back up to its 18mbit/s normal speed, which I think has to do with latency. Lots of the Mac OS standalone programs that do this kind of thing, like Link Conditioner or Speed Limit, add configurable latency to requests as well.