Transparently Rate Limiting ANY Connection that is Already In-Progress

Let's say I begin a large download, and (about an hour into it) I find that it is consuming too much of my overall bandwidth. Is there any way to rate limit that particular connection "after the fact"? I've seen tools that will allow you to control a download's speed if you begin the download using those tools, but I'm interesting a way to rate limit a download of ANY connection (even if it is already in progress). How can this be done?