Bandwidth throttling software
Solution 1:
This was asked (kinda) on SO before. The question was asking for a Firefox plugin, which we later found that Firefox Throttle could simulate slow internet speeds. Several other options were given also - the author of the question specifically desired a plugin, which is why the plugin was accepted as opposed to an alternative.
Solution 2:
If you're using linux you can use trickle, which will do what you want.