Last week i needed a possibility to simulate a very slow connection for testing the behavior of a web application. I found a nice feature integrated in Fiddler2.
Rules –> Performance –> Simulate Modem Speeds
It works very well for testing loadingscreens, progressbars, uploads and so on.
cheers
--- Update 2015-11-04
Scott Hanselman also wrote a nice post about this topic incl. integrated Chrome feature (Link).