How do you lower the mimic low bandwidth settings for testing HTML rendering on slow computers? I'm using Safari 4 on a Mac.
You're probably looking for traffic shaping, in MacOS X it's included in the kernel. Take a look at http://www.macgeekery.com/hacks/software/traffic_shaping_in_mac_os_x
You could set up a local webserver (ie Apache) and use the traffic shaping capabilities thereof.
An article on Apache throttling
I guess most popular webservers support this.
edit:Typos
There is a program by Intrarts called "throttled", and the command-line version seems to be free (price):
throttled is a bandwidth shaping application for Mac OS X and FreeBSD which allows you to cap your upstream bandwidth, prioritize ACK packets, and keep your download speeds high even when your server is sending out at full speed.
...which seems to be a little tangential to your needs, but might be able to be configured to do what you want.
I'm a big fan of using the Charles Web Debugging Proxy. It allows you to throttle your own bandwidth to simulate different connection speeds (and a whole mess of other stuff, too). Highly recommended.
I wrote a post on using waterroof to limit bandwidth a while ago.
It uses ipfw under the covers, but has a nice UI for this kind of thing.
Found the SpeedLimit preference tool mentioned in another SO answer. Works for me and is very unobtrusive to use (no application, no command line, no Java).