[Fiddler Web Debugger][1] is an excellent HTTP proxy debugging tool for Windows that includes a **modem speed simulation** feature *(Main menu > Rules > Performance > simulate modem speed)*.<br>  Fiddler is **freeware**.


I also like [Charles Web Debugging Proxy][2], a similar tool.  Charles can also **throttle the connection speed**.  Charles is commercial software, but has a free trial available.  What I like most about Charles is that it is **cross-platform**: Being Java-based, it can run on Windows, Mac OS X, and Linux.  (You didn't mention which platform you're on, but it's probably one of those :-)

  [1]: http://www.fiddler2.com/fiddler2/
  [2]: http://www.charlesproxy.com/