semi-ot: network monitoring tools

["John Levine" <johnl () iecc com>]

I was talking to a bunch of people who run ISPs and other networks in
LDCs (yes, including Nigeria) and someone asked about monitoring tools
to watch traffic on his network so he can get advance warning of dodgy
customers and prevent complaints and blacklisting.

These people are plenty smart, but don't have a lot of money.
Suggestions welcome.

R's,
John