A time ago I played with a linux box and 4 cable modem (each had a different IP). It just were a load balancing with the kernel routing tables. I remember that BitTorrent was the sole thing where I could get the total speed being the sum of each link. The problem was that the routes are cached by session (dhost, dport I think) so parallel HTTP/FTP download of a file would go through the same link. However, routes expiring gave me problems with some services which doesn't like your IP to change.