Two applications perform network i/o concurrently. For instance, Firefox and Download Accelerator.
The idea is to let serve the queue of packets of Firefox prior to serving those of Download Accelerator. (Well, almost all the time — allow a portion of those of Download Accelerator to come through in order to avoid breaking the connections)
It is no problem to configure them both to use a local proxy (maybe with different credentials).
Which local proxy, a firewall or ... you name it ... can do this ?