Depending upon your requirement , you can choose either PRTG Network Monitor or Traffic Grapher

Comparison between PRTG Network Monitor 7 PRTG Traffic Grapher 6

PRTG Network Monitor :

It monitors your network using a whole range of technologies and assures the availability of network components and measures traffic and usage. It saves costs by avoiding outages, optimizing connections, saving time and controlling service level agreements (SLAs).

For more details: http://www.paessler.com/prtg7/

PRTG Traffic Grapher:

PRTG (Paessler Router Traffic Grapher) is an easy to use software that monitors bandwidth usage and many other network parameters via SNMP, Packet Sniffing, or Cisco NetFlow. It allows you to quickly and easily set up and run a monitoring station for networks. With just a few mouse clicks you can log the amount of data flowing through routers and leased lines, monitor CPU utilization, analyze the traffic by type, or check disk space usage.
The most common usage is monitoring the bandwidth usage of leased lines, routers, and firewalls via SNMP, packet sniffing, or NetFlow. But you can also monitor many other aspects of servers, managed switches, printers, and other network components, as long as they are SNMP enabled.
For more details: http://www.paessler.com/prtg6

Installing PRTG

PRTG Network Monitor is optimized for easy installation, configuration and use. It allows you to set up your complete network monitoring scenario within minutes.Simply download the file and double-click it to start installation. The process is simple and consists entirely of answering routine questions, such as the installation path.
What is SNMP?

It stands for Simple Network Management Protocol, and is a set of standards for communication with devices in a TCP/IP network. With the help of an SNMP tool, it is possible to monitor and control network devices such as hosts, routers and switches.