One of my clients recently ran across a great tool for simulating a low-bandwidth connection to a site.  In his case, he was trying to verify that media could properly stream for users accessing the site via a slower DSL connection.

The solution — NetLimiter (http://www.netlimiter.com).

NetLimiter is an ultimate internet traffic control and monitoring tool designed for Windows. You can use NetLimiter to set download/upload transfer rate limits for applications or even single connection and monitor their internet traffic.

Along with this unique feature, Netlimiter offers comprehensive set of internet statistical tools. It includes real-time traffic measurement and long-term per-application internet traffic statistics

So that’s pretty neat!  You can open up Internet Explorer set to only use 45kbps and see how your website loads on dial-up.  They even have a version out that’s compatible with Vista.  It’s relatively cheap too.  $17 for the version that lets you limit your bandwidth, or $30 for the version that also adds the ability to do advanced rules, traffic charting/reporting, etc.