If you are running Linux, the following command is really useful for this:
trickle -s -d 50 -w 100 firefox
The -s tells the command to run standalone, the -d 50 tells it to limit bandwidth to 50 KB/s, the -w 100 set the peak detection window size to 100 KB. firefox tells the command to start firefox with all of this rate limiting applied to any sites it attempts to load.
Update
Chrome 38 is out now and includes throttling. To find it, bring up the Developer Tools: Ctrl+Shift+I does it on my machine, otherwise Menu->More Tools->Developer Tools will bring you there.
Then Toggle Device Mode by clicking the phone in the upper left of the Developer Tools Panel (see the tooltip below).
Then activate throttling like so.
If you find this a bit clunky, my suggestion above works for both Chrome and Firefox.