Server/Virtual Host Bandwidth Management

[Gavin Young <gavin young clendons co nz>]

We have quite a few web servers providing different services to
different clients, some dedicated servers and some with virtual hosts. 

Is anyone aware of a bandwidth management package that can at least
monitor throughput on a server and/or virtual host?

Ideally it would be centralised and you could actually limit servers
and/or virtual hosts to a data cap and/or speed.

What do other people use? Any help, comments would be much appreciated.

Gavin