Solar Wind's Orion can monitor almost everything and send email alerts, as well as store historical data. You can monitor specific interfaces on your routers and see what % utilization they are at, or get a Mbps number as well.
If you are looking more for, is customer A using more bandwidth than customer B, you can try Cacti. I believe Cacti is free. We use it to bill our customers with by how much bandwidth they use.
Below is a Cacti chart for one of our customers. You can see that they use multiple IPs and multiple ports on our routing equipment.
That is just one of several graphs that Cacti can make.
Hope this helps,
OBXandos