As the other posted suggested freedownload manager has a simple 3 level
throttle
however www.flashget.com has the best one although its not exaclty free
(after a month it has an nag screen when you start it)
if you are interested in throttling all kinds of applications take a look at www.netlimiter.com (shareware), there is a freeware monitoring version
there also.
This last app is fantastic.... I highly recomend it.