MRTG is a great tool that we use a lot - especially good if you want to offer traffic monitoring to customers. MRTG can get the stats from the customers port and display that as a GIF on your web server, accessed using the customers ID and password. Stops all those calls from customers asking if their access lines are overloaded...
It is mostly used for traffic monitoring, but can work with any SNMP variable. In the latest release it can monitor/display round trip delays as well.
Netflow has also been mentioned on this thread. The drawback is that Netflow only works on cisco 72xx and 75xx platforms.