Carl,
Probably the best tool around (an industry standard tool perhaps?) is MRTG. Another extremely useful tool, written by the same guy is SmokePing.
MRTG will graph your network utilisation. Smokeping will graph your network latency. Using those two tools alone will tell you which networks are having problems.
For a Flight Deck utility, have a look at Nagios.
For router/switch configuration backups, look at Rancid.
All of them are Open Source :)
Cheers,
- bec
