Inside any network regardess of size, it's important to have a traffic monitor. It hekos to detect faults and network usage. The software that I know and have used are:

MRTG - Multi Router Traffic Grapher

This Open Source software (available at http://www.mrtg.org/) can be installed over UNIX (common) and Windows Operating Systems to make traffic graphs with a sampling time of 5 minutes minimum using a default configuration (example) taking data from several sources, generally from SNMP agent. Between each sampling, the software generates traffic graphs, this makes significant load to the system so it may limit a large scale implementation. Also due to the minimum 5 minutes sampling, it's hard to detect some faults that happed between each sampling.

RRD Tools - Round Robin Database Tools

This is an Open Source software available at this URL. As is, this is not a traffic monitoring software, but can be used with MRTG to give more monitoring load capacity by disabling the traffic graphs generation each 5 minutes and making the data stored in a database (with XML export option). This approach makes MRTG more efficient and allows monitoring sampling time from 1 minute. Also, this software have modules to generate graphs, but generally, this is used by another third-party frontend.

Routers2.cgi

This is a Open Source frontend (available here) that works with CGI, PERL RRD Tools and MRTG to make traffic graphs on demands using an organized and customized layout.

STG - SNMP Traffic Grapher

This is a freeware software that runs from Windows 98, and it was developed as an add-on for MRTG. This make realtime graphs using two input value (generally in/out) using counters available with any SNMP agent. The sampling time is from 100 miliseconds, but in the reality, it's usefull to set from 500 ms to 5000 miliseconds (5 seconds). Also, it can save the sampling to a CVS file and save the monitor config to be used later. This is a very handy software because it allow to detect fault very quickly without waiting to MRTG graphs. The software can be downloaded from http://leonidvm.chat.ru/

PRTG Traffic Grapher

This is a commercial software (there is a limited freeware edition) that can be used as a complement to STG but with more options, like web server to see the graphs from any browser, historical data, etc.. (example). This software can be downloaded from http://www.paessler.com/prtg/