Solution 1:

If you use a browser to simulate that, you can use Firefox Throttle, Internet Explorer Throttle or Fiddler.

Solution 2:

Using linux you have a software called trickle that launches a process and limits its bandwidth usage.

Solution 3:

I would check out Charles Proxy. It has bandwidth throttling and is cross-platform with a very clean interface.

Solution 4:

This is free and simple for Mac OS X and works with the every browser: speedlimit