There is a program called trickle. It is a portable lightweight userspace bandwidth shaper. It can run in collaborative mode (together with trickled) or in stand alone mode.
You may wonder – why should I shutdown the Linux box box automatically? It depends upon your situation. For example, your downloading couple of tar balls and you want to go home. You can schedule a job to shutdown Linux after downloading is completed. Linux/UNIX/BSD/OS X comes with at and cron commands to automate task. Almost all common task can be automated using at command.