Simulating Slow Internet Connection

Starting with Chrome 38 you can do this without any plugins. Just click inspect element (or F12 hotkey), then click on "toggle device mod" and you will see something like this:

Among many other features it allows you to simulate specific internet connection (3G, GPRS)

P.S. for people who try to limit the upload speed. Sadly at the current time it is not possible.

P.S.2 now you do not need to toggle anything. Throttling panel is available right from the network panel.

Note that while clicking on the No throttling you can create your custom throttling options.

If you're running windows, fiddler is a great tool. It has a setting to simulate modem speed, and for someone who wants more control has a plugin to add latency to each request.

I prefer using a tool like this to putting latency code in my application as it is a much more realistic simulation, as well as not making me design or code the actual bits. The best code is code I don't have to write.

ADDED: This article at Pavel Donchev's blog on Software Technologies shows how to create custom simulated speeds: Limiting your Internet connection speed with Fiddler.