is there anyway to throttle connection speed (bandwidth) using modern browsers (chrome, ff, ie, safari) for testing?
Edit (2017-04-09): The major browsers have this functionality built-in by now:
Original response follows for reference, it might still be useful in some scenarios.
I used NetLimiter at some point - it works with all applications and can assign individual bandwidth limits for them.
Note that while this might be a good approximation for modem users (if you ever find some), it doesn't quite show you how a user somewhere in China perceives your website. The problem there isn't so much bandwidth but rather long and largely varying ping times (up to occasional packet loss). Unfortunately, I'm not aware of applications that are able to simulate random network delays and packet loss.
As of late 2014, chrome now has this built directly into the developer tools.
https://developer.chrome.com/devtools/docs/device-mode#network-conditions
Another alternative is Charles Proxy - it lets you simulate slow network connections. This approach is browser independent, just alter the browsers network configuration to use Charles as its web proxy.