MRTG vs. character of traffic

[Glenn McCalley]

OK so the data center says my traffic is unreasonable.
Turns out they may be correct.
MRTG is showing one server with a totally unreasonable level of outbound
traffic so now I know which machine to study.  Are there recommendations for
a tool to tell me what KIND of traffic, not just the gross bit rate?  I want
to find that idiot that's causing all the bandwidth.