I wrote about using bandwidth monitoring tools at home when I was testing my broadband connection a few weeks ago. I used Multi-Router Traffic Grapher (MRTG) to determine if my ISP is giving me the bandwidth that I am paying for.
In addition to MRTG, UbuntuGeek lists other bandwidth monitoring tools available on Linux. Check it out and see who is hogging your bandwidth. You'd be surprised to know that you are not maximizing your bandwidth usage. :)