NetWorx

NetWorx is a simple free tool to monitor the traffic on your network card.  It will give you a real time graph so you can watch inbound and outbound traffic. The software logs bandwidth usage and will give you hourly, daily, weekly, and monthly reports.  You can even set up custom reports to show traffic from one date to another.  You can select a specific network card or monitor all in the system. You can even have it ignore network traffic and only monitor internet bound traffic.   This is a great tool to monitor your servers internet usage so you can estimate the cost for bandwidth before you place your server in a co-location.

http://www.softperfect.com/products/networx/