Something is chewing up all of my bandwidth, is there any software to find out who and why?

All,

I work in a very small office that should not be using up all of the available bandwidth, and yet we are somehow. I've used mrtg in the past to see how much traffic is going through a device, but I have never needed to know (until now) who my (for example) top 10 or 20 bandwidth eaters are, and what they are doing. Is there any such software?

Thanks all!