Bandwidth Accounting With Unix?

LegoB writes: "I am a student at a small, under-connected college. Despite our bandwidth woes, the administration is hesitant at buying a larger pipe, feeling that our current connection is being overly taxed by things like Napster, streaming media, and other non-educational (and non-constructive) traffic. Rather than have them start limiting certain applications, I would like to propose another alternative: bandwidth accounting. I'm hoping to find that Unix, in addition to being used as a router, can also be used as a bandwdith meter. What software packages do I need to track bandwidth by time, IP, and hopefully MAC address without massive kernel hacking?"